NEW YORK • Novak Djokovic said that both he and Australian John Millman struggled to hang on in the heat, as the Serb triumphed 6-3, 6-4, 6-4 at the US Open on Wednesday to set up a semi-final clash with Japan's Kei Nishikori.

Djokovic, a two-time champion at Flushing Meadows, found it hard to cope with the humidity at Arthur Ashe Stadium but kept his cool mentally to win in straight sets.

"(I was) very tested," the 31-year-old said. "Almost three hours. It's midnight now. Credit to John for putting up a great battle.

"We were both struggling and trying to hang in there. There was a lot of sweating but you just need to survive. They weren't easy conditions but it's the same for both players.

"Changing a lot of T-shirts, shorts... He apologised for going to change (clothes) in the second set but I said, 'It's OK, I need the rest'."

Millman is used to the searing heat in his native Brisbane, but needed to leave court to change with the score level at 2-2 in the second set, telling the umpire he was unable to put the ball in his pocket because his shorts were soaked.

"These night matches the humidity goes through the roof," Millman said. "It is tricky, but it's the same for both people. You're dripping.

"But that's no excuse or anything. I'd play in a swimming pool if I got to play a quarter-final every week at a Grand Slam."

REUTERS