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 number one appeared to run out of steam as he dropped serve early in the second.
But the world number 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 number one 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.