Venus Williams tops Group B after three straight wins
Venus Williams came from behind to comfortably beat her sister Serena 5-7 6-1 6-0 and reach the semi-finals of the Tour Championships in Doha.
Serena must now beat Elena Dementieva, who defeated Dinara Safina 6-2 6-4, on Friday if she is to progress.
She won the first set in 50 minutes, but then lost concentration and seemed close to tears as defeat loomed.
Earlier, in Group A, Jelena Jankovic progressed to the last four with a 7-6 (8-6) 6-4 win over Svetlana Kuznetsova.
The defeat ended the Russian's participation in the tournament.
World number one Jankovic was far from at her best and needed treatment on a back injury at the start of the second set.
I want to do well here. I will receive some treatment and hopefully I'll be ok for the match
The injury seemed to hamper her serve, but the Serbian said she hoped to be fit to face fellow Group A qualifier Vera Zvonareva on Friday.
"It started being stiff at the end of the first set," said Jankovic. "I couldn't push on so it took a lot of pace off my serve.
"I want to do well here. I will receive some treatment and hopefully I'll be ok for the match."
Serena Williams' future in the tournament depends on the winner-takes-all match against Russia's Dementieva, but she will need to refocus after capitulating against her sister.
"I just couldn't keep a ball in play," admitted Serena, who beat Venus in the quarter-finals of the US Open in September on her way to the title. "Everything was off.
"I have never played like that before. I have never been in a situation where I can't do anything. I was very, very frustrated."
Both sisters started tentatively, but third seed Serena seemed comfortable when she broke serve in the 12th game of the first set to take the initiative.
However, Venus broke serve in the first game of the second set and an emotional Serena, who spent the rest of the match gesticulating to her coach, lost her way.
Venus went on to win 11 of the next 12 games, taking the third set with surprising ease, for her third successive win in Doha.
It was the first time on tour that a set between the Williams sisters had been won to love.
When asked what she might be able to do to help her sister's form improve, Venus said: "There is nothing I can do.
"She just has to figure it out in her head as in the other matches, even though this one was against me. So she just has to take some time and she will come out stronger."