Defending US Open champion Marin Cilic withstood a rousing Jo-Wilfried Tsonga fightback to win a fifth-set decider and reach the semi-finals in New York.
Croatia's Cilic, 26, looked to be on course for a straightforward victory after taking the first two sets.
But Frenchman Tsonga was rejuvenated after receiving treatment to his knee and levelled up against the ninth seed.
Cilic, himself with a strapped-up right ankle, recovered to win 6-4 6-4 3-6 6-7 (3-7) 6-4 in three hours 58 minutes.
The world number nine, who claimed his first career Grand Slam title at Flushing Meadows last year, had squandered three match points in the fourth set before allowing 19th seed Tsonga to win a tie-break and take it to a decider.
"That was a big mental fight after that fourth set. It got physical towards the end. It was very hot," said Cilic, who will face the winner of Tuesday night's quarter-final between Novak Djokovic and Feliciano Lopez.
Vinci into new territory
Italian Roberta Vinci reached her first Grand Slam semi-final by beating France's Kristina Mladenovic.
World number 43 Vinci, 32, triumphed against her fellow unseeded opponent 6-3 5-7 6-4 in sweltering conditions.
She will face Serena Williams in the last four after the top seed won 6-2 1-6 6-3 against older sister Venus.
Mladenovic struggled with the 34C temperatures on Arthur Ashe Stadium and received treatment in the second set.
The 22-year-old world number 40 had just survived two break points to lead 5-4 before needing treatment from her trainer, who used ice to try and cool her down.
She went on to take the second set, but looked exhausted as Vinci responded to close out the match, with the vital break coming in a 10-deuce game at 3-3.
