The Champions League began on Tuesday and if we're being honest, we know who's probably going to win. Through one-sixth of the group stage, FiveThirtyEight gives defending champion Bayern Munich a 26 percent chance of repeating, followed by Manchester City (15 percent), Barcelona (9 percent), Liverpool (8 percent) and PSG (8 percent). PSG's odds sank by three percentage points after an upset loss to Manchester United, though, and Real Madrid sank from 6 percent to 4 percent following a jarring 3-2 home loss to Shakhtar Donetsk.

Bayern more than looked the part of a favourite in Wednesday's 4-0 destruction of Atletico Madrid, but with two of what were considered the "top six favourites" losing, there's a lot of space for lower-tier contenders to advance pretty far in the draw. Besides, all but the chalkiest of tournaments produce surprises. Tottenham Hotspur made it to the final in 2019, while RB Leipzig, Lyon, Ajax, Roma and Monaco have all made somewhat unexpected runs to the semifinals in the past four years.

There are twists and turns along the way, so with that in mind, let's talk about some teams that could be responsible for some major twists to come.

The term "sleeper" can mean a lot of different things -- sleeper to win the whole thing? sleeper to advance out of the group stage? -- so let's talk about teams that fit each possible definition. It might not be likely that these teams will win the title, but they're more than capable of either knocking out your favourite team or becoming your favourite team in the months ahead.

- Stream ESPN FC Daily on ESPN+ (U.S. only)
- Predict results in ESPN's English Soccer Pick 'Em!
- ESPN+ viewer's guide: Bundesliga, Serie A, MLS, FA Cup and more

Sleeper to win the whole thing

RB Leipzig

Group: H
Title odds, per FiveThirtyEight: 4 percent
First match: beat Istanbul Basaksehir, 2-0
Next match: at Manchester United (Oct. 28)

Despite reaching the Champions League semis in August, Julian Nagelsmann's squad were stuck in Pot 3, but thanks to United's upset of PSG, FiveThirtyEight now gives the Red Bull-owned squad a better chance of advancing (63 percent) than PSG (62 percent). A win in Manchester next Wednesday would put them in perfect position to win their group.

The absence of former star Timo Werner, now with Chelsea, is likely hurting RBL's odds, but they've shown no sign of slippage without him, beginning the Bundesliga season with 10 points from four matches. They controlled Basaksehir from the opening kick and got to ease off the throttle pretty quickly.

RBL is attacking from everywhere. In five league and cup matches, seven different players have scored and 16 have logged either a key pass or an assist. Midfielder Angelino (four goals) and forward Yussuf Poulsen (three) are thriving, and the team has only gotten 45 minutes out of injured winger and captain Marcel Sabitzer thus far. We'll see if he's ready for the trip to Manchester, but RBL are again proving themselves worthy of big-boy status.

Sleeper to reach the final

Inter Milan

Group: B
Odds of reaching the final, per FiveThirtyEight: 6 percent
First match: drew with Borussia Monchengladbach, 2-2
Next match: at Shakhtar Donetsk (Oct. 27)

Antonio Conte and Inter could not take full advantage of Real Madrid's defeat, drawing with Gladbach despite creating a 3.5-1.5 xG advantage. It was unlucky, but it was also reaffirmation that this team is fun as hell.

Inter nearly won Serie A last year and have almost inarguably performed better than either Juventus or Barcelona -- two teams with far stronger betting odds -- since the coronavirus restart this summer. Their only losses in the last three months were to Sevilla in the Europa League final and to a torrid AC Milan this past Saturday.

They kept most of last year's squad intact and added both full-back Achraf Hakimi and veteran midfielder/enforcer Arturo Vidal. They were really good last year, and they have more pieces this time around.

The Nerazzurri were, along with Atalanta and RB Leipzig, easily the most proven teams in Pot 3 of the draw, and they find themselves in a chaotic group -- FiveThirtyEight gives Inter, Shakhtar and Real Madrid each between a 51-59 percent chance of advancing. A win at Shakhtar next week, however, could give them not only great odds of advancing but also a solid shot at winning their group and generating a favourable draw in the round of 16.

Sleeper to reach semifinals

Sevilla

Group: E
Odds of reaching the semis, per FiveThirtyEight: 15 percent
First match: drew with Chelsea, 0-0
Next match: vs. Rennes (Oct. 28)

I know, I know: wrong tournament. Sevilla are kings of the Europa League, having won it six times since 2006, so it's easy to simply assume they'll finish third in their group, land in the Europa knockout round and make a run.

They have a real chance to make some noise in the UCL this time around, though. They've done so before, after all -- they made the quarterfinals in 2018, barely falling to Bayern Munich, and they've now scored a road draw against their top Group E challenger in Chelsea. The Blues and Rennes both remain exciting teams, but the path to winning Group E is quite manageable.

Also... they're pretty dang good. They brought in far more than they sent out in the transfer window, they gave Bayern fits in an eventual extra-time loss in the UEFA Super Cup, and while their goalscoring form has left them a bit in La Liga play -- after scoring three goals in their league opener, they've scored just two since -- their xG figures are solid, and their passing game is sublime. And in an offense-friendly group, they have the sturdiest defense led by centre-bacls Diego Carlos and Jules Kounde.

Sleeper to reach quarterfinals

Olympiakos

Group: C
Odds of reaching the quarters, per FiveThirtyEight: 18 percent
First match: beat Marseille, 1-0
Next match: at Porto (Oct. 27)

The top of the Greek Super League features quite a few teams capable of doing minor damage in Europe, but Pedro Martins' Olympiacos have lost just a combined three league matches over the past two seasons.They took points from Tottenham Hotspur in last year's Champions League, then beat Arsenal in the Europa League knockouts before falling narrowly to Wolves. In this year's Champions League play-off round, they beat Cyprus' Omonia to qualify, and while the scoring margin (2-0) wasn't dramatically impressive, all the other stats were: they outshot Omonia 45-12, generated 4.2 xG to 0.7, etc. And now they've got three points from one match in group play.

Olympiacos are a fun mix of leathery old veterans -- former Marseille attacking midfielder Mathieu Valbuena (36), former Bayern full-back Rafinha (35), former Watford full-back Jose Holebas (36) and former Granada striker Youssef El-Arabi (33) all started against Marseille -- and peak-age talent like attacking midfielder Kostas Fortounis (28) and Wednesday's goal scorer, Ahmed Hassan (27).

This is a tough, physical and unintimidated team. Even if they can't steal points from Manchester City in Group C, they've got more than talent and identity to work their way into the knockout rounds.

Sleeper to reach the round of 16

Rennes

Group: E
Odds of advancing to the knockout round, per FiveThirtyEight: 21 percent
First match: drew with Krasnodar, 1-1
Next match: at Sevilla (Oct. 28)

Rennes missed a prime opportunity by only drawing with the weakest team in Group E, Krasnodar. They took the lead in the second half and then immediately gave it back. But hey, that kept their odds of advancing low enough to call them sleepers!

After finishing third in last year's abbreviated Ligue 1 campaign and therefore qualifying for the Champions League for the first time ever, Rennes began this season with 13 points in their first five matches before losing stars Edouard Mendy (Chelsea) and Raphinha (Leeds United) to the Premier League. The next batch of stars is already in place, however, and as they gel, they could improve as the group stages move along.

Julien Stephan's squad brought in Juventus defender Daniele Rugani and Inter Milan full-back Dalbert on loan, and new acquisition Serhou Guirassy has already scored three goals in Ligue 1 play. And they still have 17-year old phenom Eduardo Camavinga, whose presence -- and uncanny ability to do this -- makes them a must-watch team.

Whether or not they advance, their two group matches against Chelsea -- Nov. 4 in London, Nov. 24 in Rennes -- will be an absolute blessing if you enjoy things like "goals" and "fun, attacking soccer."

Sleeper to reach the round of 16

FC Salzburg

Group: A
Odds of advancing to the knockout round, per FiveThirtyEight: 32 percent
First match: drew with Lokomotiv Moscow, 2-2
Next match: at Atletico Madrid (Oct. 27)

The No. 2 team on the Red Bull totem pole held a 1.1-0.7 xG advantage over Lokomotiv on Wednesday but suffered a disappointing draw... and their odds of advancing still went up thanks to Atletico's massive loss to Bayern.

The spreadsheets love Salzburg: they're 19th in FiveThirtyEight's club rankings and 22nd at EloFootball.com. They lost Hwang Hee-chan to Leipzig -- and, of course, Takumi Minamino to Liverpool and Erling Haaland to Borussia Dortmund last January -- but the next generation of stars is ready for the UEFA spotlight. Patson Daka, 22, has scored 11 goals in just nine matches this year and generated more than half of Salzburg's xG on Wednesday. Hungarian starlet Dominik Szoboszlai (19) scored the goals of the day against Lokomotiv, and Sekou Koita (20) is excellent. And now American midfielder Brenden Aaronson (19) is coming aboard as well.

It's safe to say that your favourite club's scouting team is and has been watching Salzburg for a while, and while getting paired with Bayern and Atletico makes advancing difficult, they have the ball-control game to make things dicey for the favourites. a win next week would put them in a strong position to advance.

The ultimate sleeper

Midtjylland

Group: D
Odds of advancing to the knockout round, per FiveThirtyEight: 3 percent
First match: lost to Atalanta, 4-0
Next match: at Liverpool (Oct. 27)

Okay, no, Midtjylland probably aren't going to advance. FiveThirtyEight ranks them 136th overall, and their odds of advancing are better than only Basaksehir's. They're stuck in maybe the most aesthetically pleasing possible group with Liverpool, Atalanta and Ajax. Their goal differential is already -4. But it's a story that they're here. Call them a sleeper in our hearts.

Owned by former professional gambler and analytics lover Matthew Benham (who also owns Brentford FC), Midtjylland won its first Danish Superliga title in 2015 and lost to Manchester United in the 2016 Europa League knockout rounds. After years of knocking on the door, they finally qualified for the Champions League group stage by upsetting a solid Slavia Prague with a thrilling late surge of three goals in seven minutes.

Like Salzburg and Rennes, Midtjylland is developing a reputation as a top-notch scouting-and-development club -- which is to say, bigger clubs are ready to pounce when a younger player looks good, and the foursome of Anders Dreyer, Sory Kaba, Awer Mabil and Frank Onyeka (four shots, 0.6 xG against Atalanta) is awfully fun to watch.