LONDON - Dominic Thiem defeated Kai Nishikori 6-1, 6-4 in the first of the two games in the Leyton Hewitt Group of the ATP finals at the O2 Arena here on Thursday.

It is Thiem's first win of the tournament and has the secondary effect of meaning that Kevin Anderson (who beat both the Thiem and Nishikori in his two opening matches) is assured of a semifinal place no matter what his result is against Roger Federer later on Thursday evening.

Nishikori had beaten Thiem 6-3, 6-1 in their last meeting, but got off to a bad start as the Austrian broke his first service game.

Although the Japanese had a chance to break back in the fifth game, Thiem was able to withstand the pressure and then Nishikori hit a double fault to lose the sixth game on his serve, before Thiem served out to take the set.

Nishikori cut an unhappy figure at the end of the set and threw his racket down in frustration, although his game picked up in the second set. Thiem meanwhile was looking increasingly confident on his backhand and that shot allowed him to make the decisive move in the eighth game, to take a 4-3 lead, which quickly became 5-3.

Nishikori showed grit to hold his next service game, but Thiem was playing with confidence and blasted into a 40-0 lead in the final game and taking the first match point with an ace.

Although the win will help Thiem's confidence, the final score means that Federer will qualify for the semifinals if he can win 6 games against Anderson later in the day.