Paris - Roger Federer reached a record-equalling 31st Grand Slam semi-final on Tuesday, surviving a brutal French Open barrage from frail Argentine Juan Martin del Potro before clinching a 3-6, 6-7 (4/7), 6-2, 6-0, 6-3 triumph.
Third seed Federer, the 16-time Grand Slam title winner and 2009 champion in Paris, came back from two sets to love down for the seventh time and will take on either top seed Novak Djokovic or Jo-Wilfried Tsonga for a place in Sunday's final.
It was cruel luck on the 1.98m Del Potro who had looked on course for a straight sets win after unleashing a brutal wave of power-hitting off both wings which left Federer floundering in the first two sets.
However, his hopes were finally sabotaged by a recurrence of the knee injury which has plagued him in Paris and which left him struggling to move by the end of Tuesday's match.
"I knew Juan Martin's knee was bothering him and was trying to finish the rallies quickly," said Federer. "I knew it would be a big fight today."
Federer, playing in his 50th straight Grand Slam tournament and 32nd consecutive quarter-final at the majors, took an 11-2 career advantage over the towering Argentine into Tuesday's clash.
One of those wins came in a gripping Roland Garros semi-final in 2009 when he twice fought back from a set deficit to win in five before Del Potro gained revenge three months later with a first Grand Slam title triumph at the US Open.
The 23-year-old Argentine had lost all five meetings with the Swiss since returning after almost a year out from wrist surgery in 2011.
But despite coming onto Suzanne Lenglen court with his left knee heavily strapped, he broke Federer three times in the opening set to take the early advantage.
Federer had been struggling on first serve and on his backhand, but he managed the first break to lead 3-2 in the second set, coming out on top after a gruelling, brutal rally.
Back came Del Potro to level at 3-3 and he romped through the tie-break when Federer struggled to get any meaningful touch off another sizzling Del Potro forehand which landed at his toes.
However, Del Potro, who had spent three hours more than Federer on court getting to this stage, suddenly wilted in the third set as his often frail physical condition threatened to shatter his hopes.
He slipped 2-0 down on a double fault and at 3-1 down needed the doctor on court, a common sight throughout his Roland Garros campaign.
Four games later, Federer broke again to take the third set.
It was a similar tale in the fourth as the Swiss star romped to 6-0 with Del Potro, now struggling to move, winning just eight points.
Ninth seed Del Potro needed his knee bandaged before the decider got underway, but Federer was easily on top and took victory when a forehand return from the Argentine clipped the net and sailed out.