Jelena Jankovic and Venus Williams both progressed into the semi-finals of the WTA Championships in Doha.

Venus Williams reached the semi-finals of the Sony Ericsson Championships on Thursday with a hard-fought 5-7 6-1 6-0 win against her sister Serena to stay undefeated in three matches in Doha.

Serena Williams was in superb form in the first set and looked on course to win her second match of the tournament but she failed to maintain the momentum and Venus powered to take the next two sets, dropping just one game.

"I'm pretty excited," Venus told Eurosport. "When I go down I always seem to come back again.

She added: "I started serving a little more consistently and I tried to cut down on errors and I did that, especially in the last part of the match.

"Serena and I are two very different players. I'm more calm whereas she's quite emotional. I think she lost a little bit of focus in the third set and was focusing on the errors too much.

"I want to be fitter so I will do my best in this tournament and work hard on the fitness in the off-season."

Top seed Jelena Jankovic also advanced on Thursday with a 7-6 (8/6) 6-4 defeat of Svetlana Kuznetsova.

The victory meant the Serbian, who is 2-0 in round-robin play, topped the White Group, and she could stay there if she beats Vera Zvonareva on Friday.

She looked to be cruising in the first set but Kuznetsova battled back and eventually lost only on a tie-break. However, Jankovic's superior experience told in the second set and she comfortably wrapped up the win.