Grigor Dimitrov launched his bid for glory at the ATP World Tour Finals on Monday with a 6-3, 5-7, 7-5 victory over Dominic Thiem as Rafael Nadal prepared to start his quest for the biggest title missing from his impressive collection. A single break was enough for Bulgaria's Dimitrov to take the first set, with Thiem only managing to land 37 percent of first serves in front of a sizeable crowd at London's O2 Arena. In a tight second set, sixth seed Dimitrov earned a potentially decisive break point in the 11th game but Thiem, whose service numbers improved significantly, held firm, lashing a forehand winner down the line and edging ahead 6-5.
Austria's Thiem, the fourth seed, pressed home his advantage, making the most of some nervy play by Dimitrov to break to love and level the match at one set apiece.
In a topsy-turvy decider, Dimitrov served for the match after a single break but blew his chance as Thiem broke back to level the set at 5-5.
But the Bulgarian hit back again immediately to break his opponent and, despite squandering two match points, held his nerve to close out the match.
Later, Nadal takes on David Goffin in the second match in the Pete Sampras group, with questions over his fitness after the Spaniard admitted he was not training at 100 percent following his withdrawal from the Paris Masters with a knee injury.
The world number one has won 75 singles titles in his illustrious career, including 16 Grand Slams and 30 Masters events but he has yet to win the ATP Finals in seven previous appearances.
In the absence of Novak Djokovic and Andy Murray, all eyes are on a potential showdown in London between Nadal and Roger Federer, who started his bid for a seventh ATP Finals title with a straight-sets victory against America's Jack Sock on Sunday.
In the other match in the Boris Becker group, rising star Alexander Zverev beat Marin Cilic in three sets.
The ATP Finals feature the top eight fit male singles players and doubles teams that have accrued the most points throughout the 2017 season.
The tournament is organised on a round-robin basis, with the best two from each group progressing to the semi-finals.