Sixth seed Stefanos Tsitsipas won his opening match at the ATP Finals by edging out fourth seed and fellow debutant Daniil Medvedev in London.

The Greek 21-year-old beat the Russian 7-6 (7-5) 6-4 at the O2 Arena.

There was just one break of serve in the match with Tsitsipas, the youngest player in the tournament, earning his first win over Medvedev.

"It is one of the toughest and most important victories of my career," Tsitsipas said.

"It's not easy coming in knowing you've lost five times before [to Medvedev] but this victory means a lot to me, this crowd means a lot to me.

"[There are] Greek flags everywhere - it almost feels like I'm playing in Athens."

World number one Rafael Nadal plays defending champion Alexander Zverev later on Monday in the same group.

Novak Djokovic, who can overtake Nadal at the top of the world rankings this week, leads the other group after winning on Sunday.

The top two players in each group after the round-robin stage progress to the semi-finals on Saturday.

The men's season-ending event, which is taking place in London for the penultimate year, features the top eight players of the year.

Debutant Tsitsipas holds his nerve

Tsitsipas and Medvedev are two of four singles players aged 24 or under at this year's championships although familiar names Nadal, Djokovic and Roger Federer are the top three seeds.

Tsitsipas, who beat Federer en route to the semi-finals of this year's Australian Open, won the Next Gen event a year ago - the season-ending tournament for players aged 21 or under - and said he had "goosebumps" when making his debut in London.

"I watched this event for the first time in 2010, I used to watch on TV dreaming of potentially playing on these courts," he said.

He had lost all five of his previous matches against US Open runner-up Medvedev but was rewarded for being the more aggressive of the two players as he looked to take the initiative and move to the net regularly.

There were no breaks in the first set but Tsitsipas played better in the tie-break, earning the crucial mini-break to move 6-5 ahead by winning a gruelling point at the net with a volley.

The second set was similarly tight but Tsitsipas remained immaculate on serve - he did not face a break point in the match - and eventually broke serve to go 5-4 in front, helped by the Russian choosing not to volley a return at 30-30 that landed in.

Tsitsipas served out the match in the following game, and celebrated enthusiastically after the final point as he moved top of the early group table.