Defending champion Novak Djokovic saw off Gael Monfils in a bizarre match of fluctuating fortunes to reach his seventh US Open final in New York.
The Serb beat French 10th seed Monfils 6-3 6-2 3-6 6-2 and will face Stan Wawrinka or Kei Nishikori in the final.
Djokovic, 29, had treatment on both shoulders, while Monfils could barely move at times in punishing conditions.
“Humid” is how Djokovic described it. “I think that’s all we were thinking about and feeling.”
The world number one will attempt to win his third US Open and 13th Grand Slam title on Sunday.
Djokovic had spent only six hours and 24 minutes on court in reaching the semi-finals, thanks in part to one walkover and two retirements, and he was within sight of another swift contest against Monfils.
Reporter: "Why did you rip your shirt open?"
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He led by two sets and a break as Monfils spent much of the first hour hitting the ball at half-pace, barely moving during, as well as between, points and at one stage limping theatrically.
However, he raced forward to collect a drop shot and break back in the third set, sparking a run of five straight games.
Djokovic’s fortunes took a nosedive, first as he called for treatment to his left shoulder and then as three break-back points slipped by, and with them the set, prompting him to rip his shirt in frustration.
Monfils, 30, was back in the match but he was hardly fizzing with energy, calling for the trainer early in the fourth set, bending double at every opportunity and even drinking a Coca-Cola at one changeover.
Djokovic handed another break of serve straight back with a double fault, but after a second medical timeout – this time for his right shoulder – the two-time champion clinched victory with a winner on match point that flew past an almost stationary Monfils.
“The conditions were different from most of the days during the tournament but it was the same for both players,” said Djokovic.
“We expected it – we heard it would be warm and humid.”