( 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 were n'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 did n'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 could n'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 .
Caroline Wozniacki faces Serena Williams in the U.S. Open final . Wozniacki advances in dramatic fashion after her opponent retired . The world No. 1 will play a rematch against former champion Venus Williams, who beat her at Roland Garros last year. World No . 1 Williams crushed Russia 's Ekaterina Makarova in an hour .