Austria's Dominic Thiem edged Spain's Pablo Carreno Busta 6-3, 3-6, 6-4 here for his first victory of this year's ATP Finals.
Thiem can now book a spot in the semi-finals of this elite year-end event with a victory Friday over Belgium's David Goffin, while Carreno Busta, who replaced injured world number 1 and countryman Rafael Nadal, has been eliminated from the tournament after losing his opening match.
The fourth-ranked Thiem came into this Pete Sampras Group contest at the O2 Arena in London on Wednesday with a 4-0 career record against Carreno Busta, the world number 10, and appeared headed for a routine victory when he broke serve twice en route to an easy first-set win, Efe news reported.
The Spaniard hurt his cause with 11 backhand unforced errors in the opening set, but he came alive with his return game in the second to break Thiem once for a 2-1 lead and a second time to force a decider.
The two players traded breaks of serve over the first six games of the third set, but Thiem got a critical break in the ninth game and then finished off the match thanks to some big first serves.
Both players have been mired in a slump since the US Open, with Carreño Busta having only won one match since reaching the semi-finals in New York and Thiem having won just three (not including Davis Cup).
Bulgaria's Grigor Dimitrov, who also is in the Pete Sampras Group, has already booked a spot in the last four of this indoor hard-court tournament, while Swiss world number 2 and six-time champion Roger Federer has advanced out of the Boris Becker Group.
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Round-robin play continues Thursday in the Boris Becker Group when Federer takes on Croatia's Marin Cilic (already eliminated) and American Jack Sock plays off against German world number 3 Alexander Zverev for a spot in the semi-finals.
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