John Isner ended Juan Martin del Potro's attempt to qualify for the ATP Finals and kept himself in contention for the last spot at the end-of-season showcase by reaching the semi-finals of the Paris Masters.

Del Potro had come from nowhere to challenge for a berth in London and would have been guaranteed to seal the eighth and final place by beating Isner on Friday.

It is the world number 14 from America, though, who is still in the hunt to feature at the O2 Arena this month, after beating del Potro 6-4 6-7 (5-7) 6-4.

Defeat for Del Potro brought his season to an end, but the powerful Isner will overtake Pablo Carreno Busta and play in the prestigious ATP Finals if he wins the title on Sunday.

Isner's compatriot Jack Sock is the only other player who can qualify, but he also needs to be crowned champion in the French capital this weekend.

Del Potro was 47th in the Race to London standings at the start of the US Open and has mounted a remarkable end-of-season surge, but ultimately fell short of booking another trip to London.