Former world number one Caroline Wozniacki booked a return to the US Open final on Friday as China’s Peng Shuai retired from their semi-final a victim of cramps.

Peng was taken from the court in a wheelchair as Wozniacki was declared the victor, leading 7-6 (7/1), 4-3.

Peng had been granted a controversial mid-game medical timeout in the eighth game, left the court but returned only to be stricken again.

Wozniacki, a finalist at Flushing Meadows in 2009, will play for the title on Sunday against either two-time defending champion Serena Williams or first-time Grand Slam semi-finalist Ekaterina Makarova of Russia.

Wozniacki

Peng, in growing discomfort in the second set in the bright sunshine bathing Arthur Ashe Stadium court, was overcome in the eighth game of the second set, reeling to the back of the court where she was eventually attended by a trainer and a supervisor.

She  was  granted a confusing and controversial mid-game medical timeout as Wozniacki was left to try to stay loose hitting practice serves.

Peng returned to the court and after a break of 10 minutes played five more points before she was stricken again, finally crumpling to the court, her Grand Slam dream in tatters.

Wozniacki joined the officials at her side and soon thereafter the umpire pronounced the final score.

“It was very difficult to watch,” Wozniacki said. “You want to battle and finish it properly. I feel sorry for Peng because she played really well. I hope she’ll be OK.”

Shuai Peng holding her knee before the medical time out

Wozniacki, who shocked five-time Grand Slam champion Maria Sharapova in the fourth round, is back in a Grand Slam final for the first time since losing in the 2009 US Open final to Kim Clijsters.

The 10th-seeded Dane will play for the title on Sunday against either two-time defending champion Serena Williams or first-time Grand Slam semi-finalist Ekaterina Makarova of Russia.

“It’s incredible to be in the final, I have goosebumps,” said Wozniacki, a 22-time WTA Tour winner who hadn’t reached a major semi-final since the 2011 US Open, when she was ousted by Williams. “I’ll go to have an ice bath, then watch the other semi.”

Peng’s agony made for uncomfortable viewing.

“I feel sick to my stomach just watching this,” said tennis legend John McEnroe, commentating for US broadcaster CBS