Serena Williams can't be stopped, even when she hits the wall.

An exhausted Williams managed to hang on to beat Jelena Jankovic 6-4, 2-6, 6-4 on Saturday and will play Li Na in the final of the WTA Championships.

Williams was in obvious discomfort throughout the match, moving slowly on the court and holding her face in a towel during changeovers.

"Last night I was in my hotel and I was lying down, and I just hit a wall," said Williams, who improved to 77-4 this season. "I literally felt like I just hit a wall.

"I was so tired just even standing. I think I played a lot of tennis this year, so I don't know. After a while I just felt like a big rush of air just come out of me, and I really had to pull myself through that match. I don't know how I did it."

Li advanced to her first WTA Championships final by sweeping 2011 winner Petra Kvitova 6-4, 6-2, setting up a showdown Sunday between the two oldest players in the tournament.

It's only the second time in the history of the event that two players over 30 — Williams at 32 and Li at 31 — have made it to the last four. The season-ending tournament features the top eight players in the world.

Williams is one win from her fourth WTA Championship and 11th title of the year.

Williams will become the first female player to make $10 million in a single season; she'll collect $11.3 million as runner-up or $12.3 million as champion.

Jankovic, who said she was dealing with a cold, was unable to take advantage of Williams' condition. She wasn't convinced Williams' tiredness was entirely genuine.

"For some reason, every time she starts losing, she starts serving slower or not running for certain balls. That's something that, you know, as a player you have to pay attention to. It's not the first time when we played that she's doing this," Jankovic said.

"But, when it's an important moment, a huge serve comes up, so you have to be on alert every time. You never know if she's going to come slow or a really fast one. She's the No. 1 player in the world, and she plays so well. She's by far the best player.

"But I think, winning or losing, you have to be up there and be a good sportsman."

Jankovic also noted Williams won her previous matches in straight sets, while Jankovic spent a total of 5 hours, 41 minutes on court, about two more hours than Williams.

Williams wasted a match point and failed to serve out the match at 5-2 as Jankovic won two straight games. Williams squandered two more match points before finally closing it out with a smash.

While hitting 40 winners, Williams had the same number of unforced errors. Jankovic had 22 winners — and 22 unforced errors.

By reaching the final, Li will improve two places to a career-high No. 3 ranking. It's the highest ranking for an Asian woman.

Like Williams, the Chinese player is unbeaten in four matches. She failed to advance from the group stage in the previous two years. Li said she started the year with the goal of reaching the top three in the world.

"I catch my goal," Li said. "So pretty good ending for 2013."

She has a 1-9 career record against Williams, but will try to refuel.

"Right now I'm on E (empty)," Li said. "I'm going to try to put some gas in the tank tonight, go to the gas station and fill up and try. And I think I will be ready. I have to be. I have no choice.

"We're both in our 30s and both playing the best tennis of our career. We're still getting better."

Making her eighth appearance in the event, Williams is looking to become the first player to successfully defend the title since Justine Henin in 2007. Henin also was the last to win 10 titles in a season in 2007.