Grigor Dimitrov's ATP Finals debut ended victoriously as he overcame Dominic Thiem 6-3 5-7 7-5 at the O2 Arena on Monday.
The Bulgarian, who reached the Australian Open semi-finals and claimed his maiden ATP Masters 1000 title in Cincinnati in 2017, staked his claim as a contender to progress from Group Pete Sampras with an enthralling win.
Dimitrov passed up his first chance to serve out the match in a tense third set but made no mistake two games later to triumph in the day's opening match in London.
The 26-year-old earned a break point on Thiem's first service game but let the Austrian off the hook with a backhand into the net.
The fourth seed was not as fortunate in game six, though, as a forehand miss handed Dimitrov another opportunity which he did not let slip before wrapping up the first set by holding to love.
Thiem began to find his rhythm but was unable to capitalise on a 0-30 lead in game four of the second as his wait to engineer a first break point of the match continued.
It eventually arrived with Dimitrov serving for the match after a whipped forehand into the corner from the baseline, and the energised 24-year-old levelled at one set apiece when his opponent went wide with a shot down the line.
Dimitrov was first to break in a tight decider, rushing to the net and producing a sensational drop-shot volley to close in on victory.
Thiem applied pressure in game eight but it came to nothing, however with Dimitrov serving for the match for a second time the Austrian finally found a way through to level the decider at 5-5.
Back came Dimitrov, though, with a break to love and he made no mistake in the final game of the match, converting his third match point as Thiem fired long from the baseline.