Caroline Wozniacki has advanced to her second US Open final in dramatic and controversial fashion after Peng Shuai was forced to retire mid-match from their semi-final with severe cramping.

Wozniacki was leading 7-6 (7-1) 4-3 and serving with the advantage in the eighth game of the second set when Peng had to forfeit.

Wozniacki will play the winner of Friday’s second semi-final between world No.1 and two-time defending champion Serena Williams and Russian Ekaterina Makarova in Sunday’s title match.

Peng was left facing further medical treatment after being escorted from Arthur Ashe Stadium in a wheelchair after twice breaking down in the intense heat.

With Wozniacki down a break point trying to hold for a 5-3 advantage, Peng was afforded an “off-court evaluation” after suffering cramping in both legs.

The 28-year-old was then allowed a three-minute medical timeout before returning to the court some quarter of an hour later.

Commentating on American television, tennis great John McEnroe said Peng shouldn’t have been allowed to play on, and certainly not without a penalty.

Wozniacki won the first point upon the resumption of play when Peng returned long.

But the Dane coughed up her 10th double-fault to hand Peng another break point.

Barely able to move, Peng was unable to capitalise and collapsed for a second time shortly after.

“I feel sick to my stomach watching this,” McEnroe said before the match was finally stopped.

“This is crazy for letting this happen. It’s a black eye. It’s absurd.

“I thought I’d seen it all.”

While fellow commentators condemned the bizarre situation as “both unsafe for Peng and unfair on Wozniacki”, the Danish winner once again showed her class.

The recipient of the US Open women’s sportsmanship award last week, Wozniacki said it was a shame the match had to end in such a manner.

“You want a battle and you want to finish it off,” Wozniacki said.

“I feel very sorry for Peng.”

Wozniacki had to overcome a viral illness herself to prevail in a torrid match contested with the extreme heat policy in effect at Flushing Meadows.

The 24-year-old needed medication in the second set for a viral illness.

“I’ve been in bed for the last two days,” Wozniacki said.

“I had two days off, which was lucky.”

Wozniacki now has the chance to atone for a 2009 finals loss to Kim Clijsters.

“It’s incredible. It’s been five years. I’ve got goose bumps right now,” she said.

Contesting her first grand slam semi-final in precisely three years, Wozniacki was forced to play catch-up for most of the first set.

Peng gained the opening break of the match to grab a 4-3 lead, only to drop serve after 40 consecutive holds during the tournament in the very next game.

The world No.39 broke Wozniacki again for a 6-5 advantage, but was unable to serve out the set as the Dane struck back once more to force the tiebreaker.

But after racing through the breaker to put her nose in front for the first time, Wozniacki dropped serve for a third time to quickly fall behind 2-0 in the second set.

Yet again, though, Peng was unable to drive home her advantage, double-faulting and then dumping a backhand into the net to gift the break right back.

Wozniacki was treated at the changeover, then fought off two break points to hold for 2-2.

Opportunity lost, Peng dropped serve to love as Wozniacki surged to 4-2 before the drama really unfolded.