However, Dementieva, currently fifth in the world rankings, showed plenty of grit in the second, and stunned Williams by clinging on even as she faced match point at 5-6.

The Russian eventually took the set on a tie-break before powering into a commanding 4-0 lead in the decider, which she won 6-2 with great speed around the court and the ability to hit the winner when the opportunity came. 

It ended 3-6, 7-6, 6-2 for the Russian, meaning Dementieva has taken her opening match in the so-called “maroon group” in the round-robin contest.

Venus Williams

“I was trying to fight for every point. I was trying to stay positive. It was very important to come back and win the tie-break. I think she was a little disappointed in the beginning of the third set and gave me a little bit of an advantage, and I was able to finish the match,” Elena Dementieva told the tournament’s official website.

It means Venus now has to beat her sister Serena later today to stay in the tournament, and she promised to make it a hard time for her sibling.

“I'm going to come back tomorrow and just improve my game. That's what it's all about in round-robin tennis. It's my second time playing round-robin. Now I'll see what it's like to be in this kind of competition for real,” Venus said, following the Dementieva face-off.

Meanwhile, Serena Williams beat Russian Svetlana Kuznetsova in straight sets. She took the first on a tie break and was a bit better in the second, winning 7-5.

The other winner of the day was Victoria Azarenka of Belarus. She defeated Serb Jelena Jankovic in the white group and did it convincingly.

There was a bit of battle in the opener, which Azarenka took 6-3, but she stormed her opponent in the second, 6-2.

Jankovic will play World No. 1 Dinara Safina of Russia this evening.