(CNN) -- Defending champion Venus Williams suffered an opening round-robin defeat to Russia's Elena Dementieva on the first day of the WTA Championships in Doha on Tuesday.
She now faces a make or break clash with her sister Serena who beat Svetlana Kuznetsova 7-6 7-5 in a later match in the Maroon Group.
Defeat for Venus will almost certainly mean she exits the end of season finale, with Serena also desperate for victory as she bids to reclaim top spot in the world from Dinara Safina of Russia.
Second seed Serena saved two set points in the opening set tiebreak against French Open champion Kuznetsova before taking a crucial advantage.
A late break in the second set saw the Australian and Wimbledon champion home.
Elder sibling Venus made a confident start against Dementieva, winning the first set in 40 minutes on a single break, but the fifth seed hit back to level on a tiebreaker after trailing 3-1 and seemingly headed for defeat.
The third set was one-way traffic as seventh seed Williams made a string of errors to fall 4-0 behind and Dementieva eventually wrapped up a 3-6 7-6 6-2 victory in two hours 41 minutes.
It was her first win over Williams since 2004 and a great start to her ninth appearance in the season-ending WTA Tour finale.
"It's such a long time since I have beaten her, so I'm really happy about it," said Dementieva.
"I just fought for every point. It was very emotional."
Earlier, Viktoria Azarenka of Belarus marked her debut in the elite tournament with a straight sets 6-2 6-3 win over Jelena Jankovic of Serbia in the White Group.
Jankovic, the eighth seed, was the final qualifier after a run to the Tokyo final.
U.S. Open champion Kim Clijsters did not make the top eight, only returning to the WTA circuit in the summer before winning her second grand slam in a remarkable comeback.
The eight players are divided into two groups of four, with the top two in each section qualifying for the semifinals.
The winner will take home a hefty check for $1.5 million from a $4.5 million prize pool on Sunday.