(CNN) -- After Roger Federer's escape in the men's quarterfinals, there was yet more drama at the U.S. Open in Friday's women's semifinals.
But the scenes inside Arthur Ashe Stadium in New York weren't as riveting as the Swiss' comeback from two match points and two sets down against Gael Monfils on Thursday.
Whereas Federer walked off court as the winner, a teary Peng Shuai had to be taken off the court in a wheelchair moments after crumpling to the ground with what tournament organizers called a heat-related illness.
Trailing 7-6 4-3, China's Peng -- in her maiden grand slam singles semifinal -- retired and it gave Denmark's Caroline Wozniacki a place in the final against Serena Williams.
The resurgent Dane returns to a major final for the first time since the U.S. Open in 2009 -- she was the runner-up -- but didn't want to advance this way.
"It was very difficult," 10th-seeded Wozniacki told the crowd. "You're out here and you want to battle and finish off properly.
"I just feel sorry for Peng, she played really well. It was really hot. Unfortunately, she fell ill and I hope she'll be okay."
Already a fan favorite, former No. 1 Wozniacki likely earned more backers when she crossed the net to check on the stricken Peng.
Peng, who underwent heart surgery as a young junior, had to lean against the backboard on center court for support when she was first overcome by the pain while holding a break point in the final game of the match.
She was helped off court, where an evaluation then medical timeout ensued.
About 10 minutes later -- after Wozniacki was forced to wait for Peng without any recompense -- Peng played a few points prior to stopping and slumping to the ground.
Unable to continue, she was forced to officially retire.
"You have to feel for Peng Shuai, being in her first grand slam semifiinal and having to leave the court like that," 2013 Wimbledon champion Marion Bartoli, acting as a pundit for Britain's Sky Sports, said. "And having to go through that much pain, it's pretty difficult.
"I remind all of us how hard it is to be on the court yourself. It's you and your tennis racket, and you have to compete. You have to beat the opponent, your conditions, beat everything really and be (as fit) as possible.
"It was really painful to watch as a person and player."
Before Peng's physical issues, it was a fiercely contested battle.
Peng couldn't serve out the first set and also led by a break early in the second against Wozniacki, who revealed that she had been bed ridden herself the last couple of days.
Peng's plight must have struck a chord with Williams. Appearing woozy, Williams had to retire from the doubles at Wimbledon in July.
Her tussle against Ekaterina Makarova in the second women's semifinal lacked any drama. Williams, seeking her first grand slam title of the campaign and 18th overall, overpowered the Russian lefty 6-1 6-3.
The lone worry for the two-time defending champion came when she was broken trying to serve out the contest.
Sunday's finale figures to be close. Williams and Wozniacki met twice leading into the U.S. Open, and the current No. 1 prevailed in three tight sets on both occasions.
A victory for Williams and she would tie legends Martina Navratilova and Chris Evert in grand slam singles titles.
"She obviously wants to win and go for her first grand slam and I want to win and make a little history," Williams told the crowd.