Istanbul - Serena Williams overcame physical problems as well as emotional distress to defeat Jelena Jankovic 6-4, 2-6, 6-4 on Saturday and reach the WTA Championships final.
The world number one and defending champion, who will face China's Li Na in Sunday's final, put in a stumbling performance and twice sat weeping into her towel during change-overs.
She also committed 40 unforced errors and hit 40 winners, and allowed a 5-1 final set lead to evaporate to within one point of 5-5 before finishing the match with three trademark heavy blows.
"I'm relieved and just glad to be in the final," said Williams.
Li earlier beat Petra Kvitova 6-4, 6-2.
Sunday's final will feature two players over the age of 30.
"The oldest players have more experience on court," said Li.
"This year I was feeling more confident. I understand more what I should do on the court and off it."