Venus Williams secured her place in the semi-finals of the WTA Championships when she completed her round-robin matches with a comprehensive 5-7, 6-1, 6-0 win over her sister Serena on Thursday.
It was the first time Serena had lost a set 6-0 since the 2007 Miami final, after which she recovered to defeat Justine Henin for the title.
Venus has won all three of her matches in the maroon group, while Jelena Jankovic and Vera Zvonareva have qualified from the white group.
They both have two wins with one match to play, while the others in their group, Svetlana Kuznetsova and Ana Ivanovic, are eliminated after both losing twice.
The 18th match between the Williams sisters was an unmemorable affair but for the extraordinary capitulation by Serena.
After two exchanges of serve each, Serena saved a break point at 5-5 and then broke for the first set when Venus netted a backhand volley. Venus then broke to lead 1-0 in the second, before winning 10 consecutive games from 2-1.
The momentum might have changed in the opening game of the final set but Serena failed to convert four break points. A succession of errors followed and it was only in the final game that she was competitive again, fighting off three match points before Venus won on her fourth.
"I don't think I've ever played like that," Serena told reporters.
"I've never been in a situation where I just feel like I can't do anything. Today I couldn't serve, I couldn't hit a backhand, I couldn't hit a forehand, I couldn't even volley. This is definitely the worst match I've played this year by far."
Venus said she thought her sister had given up in the third set.
"That first game in the third was really long and I faced a lot of breakpoints," said Venus. "But after that I think she started to get a little more impatient, probably a little more upset with her performance and let it go."
Serbian world number one Jankovic defeated Kuznetsova 7-6, 6-4 in what was, after a slow start by the Russian, a very competitive match.
After rejoicing earlier this week that she was injury-free, Jankovic received treatment for a back problem during the second set and she is uncertain if she will be able to play on Friday against Zvonareva.
"It just started today during the match," said Jankovic. "I had it in the past but not recently. I dislocated something when I ran for some ball and that is the problem.
"I hope that the physios are going to help me and that I'm going to be able to play tomorrow."
Dinara Safina finished with three defeats in three matches when she was beaten 6-2, 6-4 by Elena Dementieva.