Defending champion Venus Williams was stunned by a brilliant comeback from Elena Dementieva in Doha, with the Russian earning a 6-3 6-7 6-2 success in their WTA Sony Ericsson Championship opener.

Williams looked to be cruising towards an opening victory in the round-robin format of the tuornament in Doha - as she led 6-3 3-1 at one stage.

However, plucky Russian Dementieva held on despite facing matchpoint in the second set and hit back superbly to level before dominating the decider as she claimed eight of the last ten games.

It was just a third win in 12 for Demetieva against seven-time grand slam winner Williams and a first against the American since 2004.

28-year-old Dementieva is playing in the closing tournament for the ninth time, but looked like enduring a losing start as double faults ruined her chances in the first set.

She fought back bravely though, and Williams seemed to collapse in the third set as a marathon match ended after two hours and 41 minutes of sweltering humidity in Doha.

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."

Second seed Serena Williams was made to work hard for her win, eventually beating number three seed Svetlana Kuznetsova 7-6 (8/6) 7-5 in an even contest.

The American broke four times during a match which lasted almost two hours, while her Russian opponent had three breaks of serve with four double faults proving crucial to the final score.

Victoria Azarenka made short work of Jelena Jankovic on her debut in the tournament.

The Belorussian raced to a 6-2 6-3 win over her Serbian opponent in one hour 22 minutes in the opening match of the tournament, which features the top eight players in the world.

It was Azarenka's second win over Jankovic in their fifth career meeting, and her success on her opponents' serve proved key.

Azarenka broke twice in the first set and three times in the second to pave the way for victory.

The 20-year-old won 50% of points of Jankovic's first serve and 60% on her second, carving out nine break opportunities.

She faced just two break points of her own, losing one.

Azarenka and Jankovic are in the White Group along with Russian Dinara Safina, the world number one, and Denmark's Caroline Wozniacki.

The top two players from each group advance to the semi-finals.