The Dallas Mavericks would be the second-biggest preseason long shot to win the NBA title in 35 years -- and the betting public believes they'll do it.

Approximately 80% of the bets placed this week at U.S. sportsbooks have been on the Mavericks to upset the Boston Celtics, who are only the third Eastern Conference champions to be favored in the Finals in the past 24 years.

The Celtics were -225 favorites at ESPN BET on Thursday ahead of Game 1 in Boston, with the Mavericks listed as +180 underdogs in the series. The Celtics were 6.5-point favorites in Game 1.

The Mavericks began the season at 25-1 to win the title and saw their odds drift to as long as 50-1 at some sportsbooks in January. Dallas was an underdog in 42 games during the regular season, the most of any team to reach the Finals in the past 24 years, according to ESPN Stats & Information. The Mavericks upgraded their roster at the trade deadline and have put together an impressive playoff run.

The Celtics, on the other hand, have been the consensus betting favorites to win the title all season. They were underdogs in only three games this season, tied for the fewest of any team to reach the Finals in the last 24 years. They were favored by at least seven points in each of their games during the Eastern Conference playoffs.

ESPN BET reported Thursday that 89% of the bets on the series were on Dallas. DraftKings, BetMGM and Caesars Sportsbook were reporting similar lopsided action, although the total money that's been wagered on the series winner was more evenly divided between the two teams. DraftKings reported Wednesday that 55% of the money that had been bet was on the Mavericks.

"The bigger bets and the sharper bets are on the Celtics, but there's just more bets on the Mavericks," David Lieberman, a sports trader, who oversees NBA odds for Caesars Sportsbook, told ESPN.

Boston's Jayson Tatum is the odds-on favorite to win Finals MVP at -120 at ESPN BET, followed by Dallas' Luka Doncic at +200. The Celtics' Jaylen Brown is +650, and the Mavericks' Kyrie Irving is +1,400. Doncic had attracted more bets -- and more money wagered -- than any other player at ESPN BET.