The world No 1 and defending champion appeared to be struggling for much of her 6-4, 2-6, 6-4 victory over Jelena Jankovic at the Sinan Erdem Dome, but did just enough.
The American will now bid for her fourth title at the year-end championships - and an 11th trophy of an extraordinary season - when she faces China's Li Na today.
Loading article content
Asked if she was carrying an injury, Williams said: "I just hit a wall. I was so tired. I think I just played a lot of tennis this year. After a while I just felt a big rush of air come out of me, and I really had to pull myself through that match. I don't know how I did it.
"Right now I'm on empty. I'm going to obviously try to put some gas in the tank tonight. Everyone gets pumped up for a final, so I'm hoping I'll get some adrenaline going. We'll see."
On her opponent, Williams added: "She is playing really well. She's such a good player. She moves so well. Playing her, you have to be physically, more than anything, ready to play that match. And I think I will be ready. I have to be. I have no choice."
Jankovic seemed less than impressed with Williams' display, saying: "Every time she has a lead, she's running for the balls and she's hitting those big serves.
"For some reason, every time she starts losing, she starts serving slower or not running for certain balls. She's the No 1 player in the world and she plays so well, but I think also when you play, winning or losing, you have to be up there and be a good sportsman."
Jankovic was unsure whether she would be able to play to her best because of a cold but made a great start, winning three of the first four games. Williams' fearsome serve lacked its usual bite and she was not moving well. She retrieved the break to get back to 2-3, then at the change of ends appeared to be sobbing into her towel - but returned to win 10 straight points before going on to take the first set.
Jankovic quickly took charge of the second but Williams was far from finished. Her movement improved during the decider and it was clear how much she wanted to win, something which has been one of the biggest factors in her phenomenal season.
From 1-1, Williams won four straight games. She then passed up three set points but clinched victory at the fourth time of asking.
Li, meanwhile, is guaranteed to end her season at a career high after comfortably beating Petra Kvitova 6-4, 6-2 to reach this final for the first time.
Although the highlight of Li's career remains her French Open title in 2011, victory over the Czech ensures she will overtake Maria Sharapova - absent from this tournament with a shoulder problem - and Agnieszka Radwanska to become world No 3.
She will also become the first Asian player to be ranked in the world's top three singles players, surpassing the previous high ranking of four that she shared with Japan's Kimiko Date-Krumm.
"I reached my goal - top three in the world," Li said. "That was the goal at the beginning of the year. It's a pretty good ending for 2013. Tomorrow is the last match. So I'll enjoy that and have a good vacation."
o Heather Watson's run in Poitiers came to an end in the semi-finals with defeat to Sweden's Sofia Arvidsson. The 21-year-old had won three matches at one tournament for the first time all season but could not make it four as she went down 6-4, 2-6, 6-2.
o Roger Federer survived another mid-match dip in form to take a big step towards securing his place at the ATP World Tour Finals by overcoming rising Canadian talent Vasek Pospisil 6-3, 6-7(3), 7-5 in the Basel Open semi-finals. The Swiss plays top seed Juan-Martin Del Potro in today's final after the Argentine beat unseeded Frenchman Edouard Roger-Vasselin 6-4, 4-6, 6-2.