Dane Wozniacki had to fight to win the first set of her encounter against China's Peng Shuai.

The unseeded Peng had twice broken Wozniacki's serve but was pegged back both times, ultimately losing the tiebreak 7-1.

But it was in the second set when the real drama unfolded, as the searing heat at Flushing Meadows took its toll on Peng.

She was helped off court on serve at 4-3 after holding her leg in pain, and after a 10 minute break, could only play five more points before collapsing in the heat.

Tournament officials described Peng's plight as "heat illness", and it forced her retirement from the semi-final.

It means Wozniacki is into her first Grand Slam final since the 2009 US Open.

Heatstroke forced Peng Shuai into retirement late on in the second set ©AFP/Getty Images



"It's very difficult," said the number 10 seed.

"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."

She added: "I've got goosebumps.

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

Peng said after the match: "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."

In the other semi-final, top seed Williams of the United States was in irresistible form against Russian Ekaterina Makarova.

Serena Williams remains on course for her third straight US Open title after breezing into the final ©AFP/Getty Images



She won 6-1, 6-3 in exactly one hour of play to keep on course hopes of a sixth US Open title, which would be her third in a row and her 18th career Grand Slam title.

"I'm so happy - you have no idea what it means to me," she said after booking her place in Sunday's final.

Williams has yet to drop a set and will face a resurgent Wozniacki, who had reached only one Grand Slam quarter-final since the 2012 Australian Open.

Tomorrow's action will see the men's semi-finals, with top seed Novak Djokovic of Serbia up against Kei Nishikori of Japan and Swiss Roger Federer playing Marin Cilic of Croatia.