American John Isner ended Juan Martin del Potro’s hopes of qualifying for the World Tour Finals with a 6-4, 6-7(5), 6-4 Paris Masters quarter-final victory on Friday.

Ninth seed Isner will qualify ahead of Spaniard Pablo Carreno Busta for the season-ending eight-man event in London if he lifts the title in the French capital on Sunday.

Rafael Nadal’s withdrawal with a knee injury means that last year’s runner-up Isner will face qualifier Filip Krajinovic in Saturday’s semi-finals.

The Serb has only won two matches at Grand Slam or Masters tournaments in his career.

“That’s very, very unfortunate for Rafa, and I suppose for the fans here as well,” Isner said.

“My opponent came through qualifying so has won five matches already and is playing really well. I just need to play my game and see what happens.”

Del Potro went into the match knowing that victory would secure him a place at the Tour Finals for the first time since 2013, but struggled to find his best form as a run of 25 matches since the start of the US Open in August caught up with him.

Both players were strong on serve for the majority of the opening set, but Del Potro faltered at the key moment and Isner claimed a one-set advantage by putting away a forehand volley.

Neither man managed to win more than two points in a return game in the second set, as it went to an inevitable tie-break.

Isner, who lost in the final 12 months ago to Andy Murray, went on the attack in search of the all-important mini-break, but it was Del Potro who took a 4-3 lead with a threaded backhand passing shot.

The world number 17 brought up a set point with a pair of big first serves, and Isner double-faulted to send the match into a decider.

But Del Potro appeared to be struggling with his fitness, and was broken early in the third set after Isner successfully challenged a line call when 40-15 down.

The 13th-seeded Del Potro managed to make Isner serve for the match, but the 32-year-old did just that, helped by his 15th ace, to continue his bid for a maiden Masters title.