US Open: Caroline Wozniacki into final after Peng Shuai quits

Peng Shuai was forced to retire at two sets and 4-3 down in the second

Caroline Wozniacki reached her second US Open final after China's Peng Shuai had to retire in stifling conditions.

The Dane was leading 7-6 (7-1) 4-3 when Peng began holding her leg in pain, and was then helped off the court for medical attention.

Peng returned 10 minutes later but could complete only five points before collapsing once again in the heat.

Wozniacki, the 10th seed, faces Serena Williams for the title on Sunday after the American beat Ekaterina Makarova.

Peng's condition was described by tournament officials as "heat illness", and the player was later well enough to speak to the media.

"It was a really tough match and Caroline always gets a lot of balls back," said Peng.

"When we finished the first set it was already maybe one hour or 50 minutes, and it was really hot.

"I just tried to fight, and my body was getting more and more hot and started cramping."

Wozniacki said: "It's very difficult. You're out here and you want to battle and finish it off properly.

"I feel sorry for Peng - she played really well. It was really hot out there, and it was just unfortunate she was ill."

Wozniacki had received a visit from the doctor after the third game of the second set and was given some tablets, but afterwards said: "I'm feeling pretty good.

Caroline Wozniacki shows her concern after Peng Shuai falls to the ground moments before retiring

"I've been in bed basically the last two days but fortunately we had two days off, that was a bit lucky for me."

Through to her second Grand Slam final five years after losing to Kim Clijsters at the 2009 US Open, the Dane added: "I've got goosebumps.

"It's been five years since I was in a final here and it's incredible to be back."

It was a distressing end for Peng, the doubles number three who was playing in her first Grand Slam singles semi-final.

The 28-year-old also suffered in the extreme heat of the Australian Open in January, and she left the Arthur Ashe Stadium in a wheelchair after being consoled by Wozniacki while lying at the baseline.

Peng had led in both sets only to be pegged back each time by Wozniacki, who proved much the steadier in the opening tie-break.

The Dane recovered another break early in the second and reeled off four games in a row to move within sight of victory, but she still had work to do leading 4-3 on serve.

It was at 30-30 that Peng suddenly began to suffer physically, hobbling to the back of the court and apologising to Wozniacki, who then double faulted.

Peng's condition was worsening and the trainer ran onto court to assist her, followed by officials, but when the Chinese player was helped off court it was not clear whether she would be penalised.

Wozniacki remained out in the heat of the court practising serves before saying to the umpire: "If she evaluates for half an hour, everyone could be ready again."

When Peng did re-emerge after 10 minutes she was clearly still struggling to move.

Despite somehow managing to fashion another break point among the five played, she collapsed at the baseline and a retirement inevitably followed.

"It was really hard to watch for me whenever I saw her collapse on the court," added Wozniacki.

"You know, tennis is great, but the health is more important. To see her struggling out there, I just wanted to make sure she was OK."

Wozniacki is into her second US Open final after losing her first in 2009

Peng Shuai had to be taken off the court in a wheelchair