The 32-year-old world number one, going for fourth title at the season-ending event, was not her usual self during the first two sets, not serving with her customary power and lacking enthusiasm to run for returns. Williams seemed close to tears at a changeover in the first set yet she went back on court and won eight straight points, but taking the set did not appear to improve her mood and her serves were even slower during the second
The 14,000 fans who filled Istanbul's Sinan Erdem dome were disappointed by Serena's blues and some even booed her although she had entered court for her match to the loudest cheers.
Williams found her rhythm in the third set and started hitting her strong serves once again to get the job done. "I just hit a wall, I was so tired. Physically I was so tired just even standing. My legs wouldn't move. My arm wouldn't go fast," Williams said.
"I think when you are doing something so much for so long throughout the year, maybe it can take its toll. At the end of the match, I was really just fighting just to stay in there and just try and get my legs to move to go to the balls.
"I'm just - just one word is "exhausted." I need to lay down for 24 hours and that's it, I think. But I don't have that time, so I'll try," said Williams, who became the first woman tennis player to earn more than $10 million (£7.2m) in prizes in a year.
"I was looking at the statistics when the tournament began. I saw I had like 70-something matches, and the person closest to me had like 50, I think Agniezska Radwanska.
"Really? I played 20 matches more than her? I thought, that's a lot! I didn't realize how much I had been going at it."