Serena Williams stayed on course to end the year as world number one when she beat Russia's Svetlana Kuznetsova 7-6, 7-5 on the opening day of the WTA Championships after sister Venus suffered a shock defeat on Tuesday.
Second-ranked Serena, who must better Dinara Safina's performance here to snatch the No.1 spot, served 11 aces under the Khalifa Stadium floodlights to get the better of the French Open champion who led 6-4 in the first-set tiebreak.
She will play Venus in her second round-robin match on Wednesday knowing that victory would almost certainly eliminate her older sibling who lost 3-6, 7-6, 6-2 to Elena Dementieva, the third Russian competing at the season-ending showpiece, in the day's outstanding contest.
In the opening match in White Group, former world number one Jelena Jankovic lost 6-2, 6-3 to tournament debutant Victoria Azarenka of Belarus. World number one Safina begins her tournament against Jankovic on Wednesday while Azarenka plays Denmark's Caroline Wozniacki.
Eleven-times grand slam champion Serena, bidding to end the year top of the WTA rankings for the first time since 2002, was far from her best in a scrappy slug-fest against Kuznetsova.
With the match starting around 10pm local time, only a sprinkling of fans remained in the 7,000-seater Centre Court and the subdued atmosphere appeared to affect both players as they regularly found the net in high humidity.
Kuznetsova looked poised to win the opening set when she came back from 1-3 down in the tiebreak to lead 6-4 but a Williams winner and an ace helped the American snuff out the danger and she won the next two points to move ahead.
"At that point I thought I had ruined it for myself because I missed an easy shot to go 4-1 up in the breaker," Williams, who needed treatment on her ankle in the second set, told reporters.
World number three Kuznetsova, yet to survive the round-robin stage in four appearances at the Championships, gifted Serena a break of serve at 5-5 in a tight second set with a double-fault, effectively sealing her fate.
Both Williams sisters should have been celebrating victories but Venus, the defending champion, was thrown off track by an inspired Dementieva who ended a five-year wait for only her third career victory against the American in 12 meetings.
Twice Venus was a break up in the second set, having strolled through the first, and she served for the match at 6-5 only for Dementieva to break back with a searing backhand pass.
A Venus double-fault handed Dementieva the second set tiebreak and the Russian, making her ninth appearance at the Championships in 10 years, did not look back.
"The last time I beat her was a long time ago. After last match we played in Stanford, I was feeling so miserable," Dementieva told reporters. "I was really looking forward to play against Venus one more time."
Venus, who looked troubled by the high humidity in the third set, said she let her opponent off the hook.
"I had a lot of opportunities to have an even bigger lead," said the seven-times grand slam champion. "But I didn't put the ball in the court. That doesn't help."
While the second match of the day enthralled the crowd, the opening encounter rarely resembled a contest.
Jankovic, one of three players to top the world rankings this year but who was the last of the eight players to qualify for Doha, looked miserable as she sprayed 33 errors.
"I basically gave her everything," the downcast 24-year-old told reporters. "I beat myself. That was really unfortunately the case. My game was completely off. I was a shadow of myself."
Members of Qatar's ruling Al-Thani family watched from the Royal Box as the shrieking 20-year-old Azarenka sped into a 6-2 3-1 lead before Jankovic sparked briefly into life.
The revival did not last, however, as Azarenka profited from more wayward Jankovic shots and, after coming through an eighth game that went to five deuces, she served out for victory.
