Daniil Medvedev got his ATP Finals campaign off to the best possible start as he beat Alex Zverev 6-3, 6-4 in his first match.
Medvedev, currently ranked fourth, is expected to pose the biggest threat to pre-tournament favourites Novak Djokovic and Rafael Nadal.
Medvedev went home empty-handed from his debut at the season-ender last year, losing all three matches.
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But after an error-strewn opening to the match from both players, the 24-year-old dominated the 2018 champion who again struggled with his shaky serve.
Medvedev dumped a second serve in the net to lose the opening game but Zverev served three consecutive double-faults in the next game on the way to handing the break back.
Some brutal baseline exchanges took their toll on both players and Zverev worked overtime trying to stay level at 2-3, only two throw away the game with consecutive double-faults.
From then on Medvedev was too solid for the German, taking the first set with an ace and breaking in the seventh game of the second set as Zverev again wavered.
Zverev won only four of 19 second serve points in the match - a statistic that ultimately proved costly as the wily Medvedev closed out the victory.
Medvedev will face world number one Djokovic on Wednesday. Djokovic opened his quest for a sixth ATP Finals title with a crushing straight-sets defeat of Diego Schwartzman.
Additional reporting from Reuters
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