Paris - American John Isner sealed his place in the semi-finals of the Paris Masters on Friday with a 7-6 (8/6), 4-6, 6-4 win over compatriot Jack Sock.
Isner and Sock, who teamed up to win the doubles in Shanghai last month, traded crunching blows as the first set headed to a tie-break, but the latter was made to pay after failing to convert a set point.
Sock was playing his 33rd match - singles and doubles combined - in as many days and the American No 1 appeared to run out of steam as he dropped serve early in the second.
But the world No 24 rebounded by breaking Isner's serve for the first time this week and then repeated the trick once more to force a third set.
Isner raced 4-0 ahead in the decider, and although Sock fought gamely to claw to within a break, his rally fell short this time.
Novak Djokovic, battling to hold onto his world No 1 ranking from the fast-charging Andy Murray, takes on Marin Cilic later.
Djokovic has never lost to the Croat in 14 previous meetings, but Cilic is a man in form after booking his ticket to the prestigious November 13-20 Tour finals in London.
The Serb is a record four-time champion in Paris and will extend his 122-week run at the summit if he reaches Sunday's final.
Murray has drastically cut the gap with titles in Beijing, Shanghai and Vienna and could supplant Djokovic with a first Paris Masters crown if his rival falls beforehand.
The Scot takes on 2005 champion Tomas Berdych in the quarter-finals with the Czech needing to win to stay in the race for the final London berth.
Results from the fifth day of the ATP Paris Masters on Friday (x denotes seeding):
John Isner (USA) bt Jack Sock (USA) 7-6 (8/6), 4-6, 6-4