Maria Sharapova came from a set down to win the season-ending WTA Tour Championships in Los Angeles.
The Russian beat Serena Williams 4-6 6-2 6-4 in a memorable see-saw match.
Her hopes were raised when Williams was left severely restricted by a stomach muscle injury she picked up late in the second set.
Despite being unable to serve with any power in the final set, she raced into a four-game lead before Sharapova found her groove to win six in a row.
Williams edged a tight first set with a late break of serve and, after breaking again early in the second set, it seemed as if the result would be a formality.
Sharapova battled back to level with some flourishing groundstrokes before going on to take the second set 6-2.
Despite being limited to little more than a lobbed serve at times, Williams notched up a 4-0 lead in the deciding set.
However, Sharapova suddenly refound her form and raced to victory.
"I'm still in shock," Sharapova said after securing the victory in one hour and 46 minutes.
"I can't believe the way I pulled it out. It's very unreal the way I stuck in there. It's been an amazing year.
"I could tell she was having some problems with her serve, but on the ground strokes she was teeing off on absolutely everything.
"She figured she can't do anything from her serve so she better do it in the rallies. I couldn't do anything."
Williams, who felt the injury at the start of the match, considered quitting and also toyed with serving underhand, but dismissed it as "too tricky".
"I felt something in my stomach in the first game but I thought it was just a stitch and once I got into the rallies it was much better," she revealed.
Williams also feared she could make the injury worse by playing in pain.
"I was thinking I don't want to be out for six months, it's not worth it with a new year coming up," she added.