Samantha Stosur missed a chance to make the semi-finals of the WTA Tour Championships for the third straight time after losing her opening match in Istanbul.
Stosur, who was a late call-up for the season-ending tournament after defending champion Petra Kvitova withdrew with illness, went down 6-3, 2-6, 6-0 to Italian seventh seed Sara Errani.
The Aussie needed to win both of her White Group matches to progress, with her next encounter against Maria Sharapova now effectively a dead rubber.
Stosur hit 26 more winners but also 24 more unforced errors in an erratic display which ended with a double fault on match point.
"I really wanted to try and play a good match tonight and be at least one match up, but obviously the result wasn't quite that way," Stosur told the WTA website.
"It was a little bit up and down - in the second set I got off to a good lead and then was able to hold onto that and win the set, and then I had a break point in the first game of the third and missed a return, and then it was like all the energy went out of me.
"I certainly dug myself into too much of a hole to try and get out."
Errani faces a must-win match against Poland's Agnieszka Radwanska if she is to join Sharapova in advancing from the group.
Meanwhile, Serena Williams reached the semis with what is fast becoming a routine defeat of world number one Victoria Azarenka.
The Wimbledon and US Open champion recorded a 6-4, 6-4 win, her fifth triumph over the Belarusian this year. Williams also owns an 11-1 record in their career head-to-heads.
The win was Williams' third at the tournament and while she is guaranteed to top the round robin Red Group, Azarenka must now beat China's Li Na to reach the last four.
The eighth-ranked Li ended German Angelique Kerber's participation in the event with a 6-4, 6-3 triumph.
Azarenka's power troubles nearly every other player on the WTA Tour but Williams has a substantially bigger serve than most and an equally good return.
Williams was again let down by her serving, but she still managed 35 winners to just 15 for Azarenka, broke her five times and won the match when Azarenka doubled faulted for the ninth time.
"I had too many opportunities that I didn't use," Azarenka said.
"She played really well. I didn't execute well enough. For me, the key word today was discipline, which was missing."
The 31-year-old Williams was at a lost to explain her dominance over Azarenka.
"Maybe I'm doing the right things at the right time." Williams said. "I don't know what the edge is."
Azarenka admitted she struggled mentally against Williams but fancied one more match against her before the year was done.
"She's playing great tennis," Azarenka said.
"She's letting her tennis speak for itself. My biggest opponent is her right now. I'll do everything I can. I might still have a chance to play against her in this tournament [in the final], so we'll see."
ABC/ReutersTags: tennis, sport, turkey, australia First posted October 26, 2012 09:32:51