It was a very strange performance from the world number one and defending champion, who for much of the 6-4 2-6 6-4 victory seemed barely able to run for the ball.
But Williams insisted she was not injured, merely feeling the effects of a season where she has played more matches and won more tournaments than in any other year of her career.
Her trophy count for 2013 could reach 11 on Sunday when she takes on China's Li Na, who was a much easier 6-4 6-2 winner over Petra Kvitova.
"I just hit a wall," said Williams. "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."
She had breezed through the group stages at the Sinan Erdem Dome but it was immediately obvious this was a different Williams.
At the change of ends after the fifth game, she appeared to be sobbing into her towel, but that did not stop her winning the next 10 points and eventually the first set.
Jankovic hit back to level but in the decider the American began to look more like her usual self only to then have trouble finishing the match off.
Williams saw a match point disappear at 5-2 and then two more in the 10th game, and Jankovic was a point away from clawing her way back to 5-5 before the defending champion finally clinched victory.
Williams has only lost four matches all season, but she knows she must find some energy from somewhere if she is to preserve that record.
She said: "Right now I'm on E (empty). We'll see. I'm going to obviously try to put some gas in the tank tonight.
"Tomorrow is the final so obviously everyone gets pumped up. I'm hoping I'll get some adrenaline going.
"She (Li) is playing really well. She's such a good player. She moves so well. So playing her you have to physically, more than anything, be 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.
"It's not the first time when we played that she's doing this. That's I think her way of playing, or maybe when she plays against me. I don't know.
"She's the number one 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."
Li is through to the final for the first time and her victory over Kvitova guaranteed she will overtake Maria Sharapova - absent from this tournament with a shoulder problem - and Agnieszka Radwanska to be world number three.
Additionally, Li will become the first Asian player ever 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.
She said: "I reached my goal - top three in the world. That was the goal at the beginning of the year."
Although the highlight of Li's career remains her French Open title in 2011, this season has been by far her most consistent.
Her record against Williams does not offer great encouragement, though, with the 31-year-old having won just one of their nine previous meetings.
In their last clash in the semi-finals of the US Open, Li won three games, but she is determined not to lose the mental battle this time.
She said: "I think at the US Open I 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 what she does. I have to try to play my game."