China's Li Na will meet Serena Williams in the final of the WTA championships after beating Czech Petra Kvitova 6-4, 6-2 on Saturday, becoming the highest-ranked player ever from an Asian country.
Title holder Williams struggled early on with her own game before overcoming Jelena Jankovic of Serbia 6-4, 2-6, 6-4 in the other semi-final to set up a clash between two players over 30.
Li's victory means she will climb to third in the rankings and can finish the season on a high in her first WTA Championships final on Sunday.
"It's awesome, because we're both in our 30s and, you know, I think we're both playing the best tennis of our career. We're still getting better," Williams said.
"I personally think I can do better. I'm sure she believes she can as well and it's really good to have a peer right next to you your same age doing just as good, so it feels good."
The 32-year-old world number one, going for fourth title at the season-ending event, was not her usual self during the first two sets, not serving with her customary power and lacking enthusiasm to run for returns.
Williams seemed close to tears at a changeover in the first set yet she went back on court and won eight straight points, but taking the set did not appear to improve her mood and her serves were even slower during the second.
The 14,000 fans who filled Istanbul's Sinan Erdem dome were disappointed by Serena's blues and some even booed her although she had entered court for her match to the loudest cheers.
Williams found her rhythm in the third set and started hitting her strong serves once again to get the job done.
"I just hit a wall, I was so tired. Physically I was so tired just even standing... My legs wouldn't move. My arm wouldn't go fast," Williams said.
"I think when you are doing something so much for so long throughout the year, maybe it can take its toll... At the end of the match, I was really just fighting just to stay in there and just try and get my legs to move to go to the balls.
"I'm just - just one word is "exhausted." I need to lay down for 24 hours and that's it, I think. But I don't have that time, so I'll try," said Williams, who became the first woman tennis player to earn more than $10 million in prizes in a year.
"I was looking at the statistics when the tournament began. I saw I had like 70-something matches, and the person closest to me had like 50, I think Agniezska Radwanska.
"Really? I played 20 matches more than her? I thought, that's a lot! I didn't realize how much I had been going at it."
Li, who took only three games when she lost to Williams at Flushing Meadows in September, said: "At the U.S. Open (semi-final) I had already lost the match before I came to the court.
"Maybe now I have to try to focus on what I should do on the court, not focus on what she does, so I have to try to play my game and not follow her," the 31-year-old said.
Her coach Carlos Rodriguez said Li needed to show more self-confidence to go out and enjoy her matches but she explained it was a problem dating back to her beginnings as a player.
"I always try, but sometimes it doesn't work, you know. And also, I think when I was growing up, I did not learn to feel that confidence, I cannot show how good I am, because when I was starting to play tennis, when I won a tournament, the coach was so tough," she said.
"If I won a tournament and the next day I was training, if I made a mistake, for sure she would say, 'What? You just won one tournament. Are you thinking you can win another one? Why don't you train harder?'
"I think she always punched me at the time, so that's why. I didn't have the confidence to show how good I am."
Image: Li Na
Photograph: Dean Mouhtaropoulos/Getty Images