The German Alexander Zverev took his place in the semi-finals of the ATP Finals with a win in a row, which his American opponent John Isner was excluded from the season end.
Both men dominated their service matches, but Zverev fought his way through the first set before breaking at the end of the second break and beating 6-6 (7: 5) 6-3 in London.
Third seed Zverev will face Roger Federer in the semi-final on Saturday.
Zverev's win means that Novak Djokovic is the winner in the group.
The Serbian world classic, who will face Marin Cilic in his last match on Friday, will face South Africa's Kevin Anderson on Saturday.