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.
Li Na advanced to her first WTA Championships final by sweeping past 2011 winner Petra Kvitova 6-4, 6-2, setting up a showdown between the two oldest players in the tournament.
Williams was in obvious discomfort throughout the match, moving poorly on court, walking slowly 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. I literally felt like I just hit a wall," Williams said.
"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."
Jankovic, who said she was battling a cold herself, was unable to take advantage of Williams' weakness and the American is one win from her fourth WTA Championship and 11th title of the year. Williams has won 77 of 81 matches this year.
Jankovic wasn't convinced Williams' tiredness was 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 pointed out that Williams had won all of her previous matches in straight sets - and Jankovic had spent a total of five hours, 41 minutes on court, more than two hours more than Williams. Williams also had Friday off, while Jankovic played.
Williams wasted a match point and failed to serve out the match at 5-2 and Jankovic won two straight games. Williams squandered two more match points before finally closing it out with a smash.
"In the third set, it was 5-1 for her, and I was one point away to level it to 5-all, and she hit that ace which was such a great angle, and unfortunately I lost 6-4 in the third," said the seventh-seeded Jankovic, who is back in the top 10 after three years.
While hitting 40 winners, Williams had the identical number of unforced errors. Jankovic had 22 winners - and 22 unforced errors.
By reaching the final, Li made sure of rising to a career-high No. 3 ranking, a jump of two places and the highest ranking for an Asian woman.
Li said she started the year with the goal of making it into the top three.
"I catch my goal, lucky or happy in the last tournament I make it, so pretty good ending for 2013," the Chinese said.
Like Williams, Li is unbeaten in four matches, after failing to advance from the group stage in the previous two years. But she is 1-9 against Williams in previous matches.
Williams, 32, and Li, 31, are the two oldest players in the elite, eight-woman tournament, while the fifth-seeded Kvitova is the youngest at 23.
"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.
"I'm really proud that I was even able to stay out there today. And, you know, tomorrow is the final, so everyone gets pumped up.
"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 was also the last to win 10 titles in a season, in 2007.
And she is the first woman player to make $10 million in a single season; as a runner-up she will have collected $11.3 million, as champion, $12.3 million.