Five years after her first appearance in the U.S. Open final, Caroline Wozniacki is back again.

She would have liked to be explaining how she won her semifinal match against Peng Shuai Friday at Arthur Ashe Stadium, how her shots made the difference. Instead, she was expressing her feelings for Peng.

Exhibiting what officially was called "heat illness" and looked very much like full-body cramps, Peng was taken off by wheelchair with Wozniacki leading 7-6 (1), 4-3. Officially, she had retired.

It was an excruciating final 20 minutes. Peng started to exhibit signs of distress in her legs at 30-30 in the eighth game of the second set with Wozniacki serving.

Wozniacki double-faulted, handing Peng a break point, but Peng staggered around the backcourt in agony. She was left leaning against the backcourt wall while the umpire called for the WTA trainer, who also had the doctor on standby.

The trainer and the doctor attended to her and had to help her off the court for medical evaluation. After several minutes, a three-minute medical timeout was declared and Peng returned, though she appeared to be severely impacted.

The pair played six points and twice Peng held a break point. But with Wozniacki holding an ad point, Peng fell to her knees. Medical personnel and Wozniacki went to her aid, but she clearly was done, and in another few minutes, it was announced that she had retired from the match.

"It was really hard for me to watch whenever I saw her collapse on the court," said Wozniacki, who also was aware that Peng had heart surgery as a child. "Tennis is great, but the health is more important."

When Peng returned, she managed to get a point. "She made a winner down the line on my return, I was like, maybe she's going to be fine to play," Wozniacki said. "But when she collapsed on the court the second time, I was like, this doesn't look very good."

With the temperature calculated at 82.4 degrees Fahrenheit, the U.S. Open extreme weather policy was in effect. That allowed for a 10-minute break between the second and third sets. The players each took a bathroom break after the first set and Peng did not seem to be suffering physical distress, but eight games later, she was in agony.

It had been a very tightly contested match until then, though Wozniacki did manage to steamroll through the tiebreak. Peng was hitting the more penetrating shots but also was making a few more errors. The second set was much of the same until Peng's illness.

Wozniacki had been fighting a cold and said she spent much of the last two days in bed. She also could empathize with Peng, having fought through cramps in Doha in 2009 while coming back to win a match.

It is important to note that Peng was said to be suffering heat illness, which allowed the long evaluation process. Had she simply been designated as cramping, she would have been able to get treatment only during a changeover and could have forfeited points or games for undue delay.

Peng addressed the media about three hours after the match and did refer to her problem as cramping. The doctor told her that she would be unlikely to compete. "I said, no, no, no, I don't want to give up," Peng said. "I want to try one more time. And then I come back . . . I know I maybe not going to stay too long, but I just want to try, you know."

Despite her 22 WTA victories, Wozniacki does not have a Grand Slam title. Her only other Grand Slam final was at the 2009 Open, where Kim Clijsters, coming out of retirement, beat her. In Sunday's final, she will face Serena Williams, coming into full form.