Germany's Alexander Zverev clinched his place in the ATP Finals semi-finals with a straight-set win which eliminated his American opponent John Isner from the season-ending event.
Both men dominated their service games, but Zverev nicked the first-set tie-break before breaking at the end of the second to win 7-6 (7-5) 6-3 in London.
Third seed Zverev will play Roger Federer in the semi-finals on Saturday.
Zverev's win means Novak Djokovic is through as group winner.
The Serb world number one, who meets Marin Cilic in his final match on Friday, will play South Africa's Kevin Anderson in the last four later on Saturday.