It wouldn't have been the way she wanted to do it but five years after her first grand slam final Caroline Wozniacki is back in the big time.

The 24-year-old advanced in New York when Peng Shuai of China, playing her first grand slam semi-final at the 37th attempt, was forced to retire late in the second set.

In hot, windy conditions, she was trailing 7-6, 4-3 when she went down initially clutching her knee and seemingly struggling to breathe.

After an off-court evaluation and a three-minute medical time-out, the 28-year-old played six more points but was unable to continue, crying as she left the court in a wheelchair.

Wozniacki, who will play Serena Williams after her 6-1, 6-3 semi-final defeat of No.17 seed Ekaterina Makarova, said: "I feel sorry for Peng because she played really well and it's hot out here. It's unfortunate she felt ill and hopefully she'll be OK."

John McEnroe, commentating for CBS, said: "She should never have been allowed back on court for a host of reasons. It's crazy they are letting this happen. It's absurd. I feel sick to my stomach watching this.

"The way the match ended took the gloss off Wozniacki's win in a match that had been littered with errors as both players struggled to cope with the wind. But to be in the final again is a superb achievement, even more as she was suffering from a virus.

Wozniacki added: "I was in bed most of the last two days. Luckily I had two days off and that really helped. I am fine now." On reaching the final, she said: "It's incredible. I have goosebumps. It's been five years and it's incredible to be back."