Donovan Mitchell poured in 40 points as the Cleveland Cavaliers sent the Boston Celtics spinning to a third straight defeat in a heavyweight clash of the Eastern Conference rivals on Monday.Boston's Grant Williams missed two free throws with less than a second left on the clock that would have given the Celtics victory before the Cavs escaped to win 118-114 in overtime.A depleted Boston missing stars Jayson Tatum, Al Horford and Robert Williams III went into Monday's game in Cleveland desperate for victory after consecutive defeats to the Brooklyn Nets and New York Knicks.But the Celtics' hopes of snapping out of their mini-slump ran into a red-hot performance from Mitchell, who nervelessly drained two free throws of his own with 5.8 seconds on the clock in the fourth to tie the game at 109-109.Boston however had a golden chance to win it at the death, sweeping up the court and winning two free throws after Lamar Stevens fouled Williams at the basket.Williams, though, fluffed both of his free throws, allowing Cleveland to take the game into overtime and pull away for victory.Mitchell could be seen exchanging words with Williams before the Boston player prepared to take his free throws, and the Cavs star admitted later he was "trying to mess with him mentally.""I wanted to find a way to get in his head a little bit," Mitchell said.It completed a battling fightback by Cleveland, who had trailed by 15 points in the third quarter before staging a fourth-quarter rally to haul themselves into contention, outscoring Boston 31-17 in the final frame.Mitchell finished with 40 points from 14-of-34 shooting, with 11 rebounds and four assists.