Venus Williams secured her place in the semifinals of the Sony Ericsson Championships with a 5-7 6-1 6-0 win over her sister Serena on Thursday.

It was the first time in eighteen meetings between the Williams sisters that one of them took a set without losing a game. Also, it was the first time Serena lost a set 6-0 since the 2007 Miami final against Justine Henin, which the American eventually won.

Today’s clash with sister Venus Serena considers her worst performance this year.

"I don't think I've ever played like that," said third-seeded Serena.

"I've never been in a situation where I just feel like I can't do anything. Today I couldn't serve, I couldn't hit a backhand, I couldn't hit a forehand, I couldn't even volley. This is definitely the worst match I've played this year by far."

In two other Thursday’s matches Jelena Jankovic beat Svetlana Kuznetsova and made it to the semifinals, while Elena Dementieva ended Dinara Safina’s 2008 tennis season in straight sets with a 6-2 6-4 win.

Besides Venus Williams and Jelena Jankovic, Vera Zvonareva has also qualified for the semifinals of the Sony Ericsson Championships. The fourth semifinalist will be either Serena Williams or Elena Dementieva. (source: Reuters, photos: Getty Images)

World No.1 and top seed Jelena Jankovic defeated Russian Svetlana Kuznetsova 7-6(6) 6-4 in the round robin phase of the Sony Ericsson Championships in Doha and ensured her spot in the semifinals of the season-ending event.

Jankovic broke serve in the very first game, and looked to be on a perfect way to claim the first set comfortably. However, after stepping up at 5-4 having lost only three previous points on serve, Jankovic’s level decreased and sixth-seeded Kuznetsova converted her first break point to level at 5-5. The Serb broke again in the following game, but was once more unable to serve out. Finally, Jankovic won the set in a tie-break. The second set progressed on serve for nine games but when Kuznetsova was left serving to stay in the tournament, she was unable to hold on.

The loss ends Kuznetsova’s participation at the 2008 Sony Ericsson Championships. The Russian made her event debut in 2004 and has qualified for the fourth time, but is yet to progress past the round robin stage.

Vera Zvonareva is the other semifinalist from the four-player White Group. Besides Kuznetsova, Ana Ivanovic is the other White Group competitor who can no longer qualify.

Additional info: During the match with Kuznetsova, Jankovic had problems with a back injury. Hopefully it will heal enough for her to play the rest of the matches with ease. (photo: Getty Images)

Both the Williams sisters won their round-robin matches on Wednesday at the season-ending Sony Ericsson Championships in Doha.

Third-seeded Serena saw her first action of the week and moved to 1-0 in the four-player Maroon Group with a 6-4 6-1 victory over Russian Dinara Safina, who is a disappointing 0-2 already this week. Safina lost to Venus Williams on Tuesday.

In another Maroon Group match, seventh-seeded Venus defeated fifth-seeded Olympic gold medalist Elena Dementieva 6-4 4-6 6-3. Venus is now 2-0 and appears well on her way to the semifinals, while Dementieva fell to 0-1.

On Thursday the Williams sisters will play each other. Serena is 9-8 all-time against her older sister, including a 2-1 record this year. The two have never met at the season-ending Championships.

Besides the clash of Venus and Serena Williams, the Sony Ericsson Championships will feature the following matches on Thursday: Jelena Jankovic vs. Svetlana Kuznetsova, Dinara Safina vs. Elena Dementieva. (photos: Getty Images)

Ana Ivanovic lost another match at the Sony Ericsson Championships in Doha – on Tuesday the French Open champion fell to fellow Serb Jelena Jankovic, while on Wednesday she lost the first career three-setter against Russian Vera Zvonareva.

Eighth-seeded Zvonareva defeated fourth-seeded Ivanovic 6-3 6-7(5) 6-4 after two hours and 28 minutes of play.

It was the second match at the 2008 Sony Ericsson Championships for both players. Zvonareva went past compatriot Svetlana Kuznetsova on Tuesday.

Ivanovic and Zvonareva have one more round-robin match in their White Group.

Just like in the match against Jankovic, Ivanovic again had breathing problems. Near the end of the first set today, Ivanovic received medical help, and I hear it was due to a virus.

Before today's clash at the Sony Ericsson Championships, the last time Ivanovic and Zvonareva met was in the final of the Generali Ladiez Linz ten days ago, and the Serb was victorious then in straight sets.

Gossip info: Fernando Verdasco was again in the stands supporting Ana. (photos: Getty Images)

American Venus Williams recovered from a 5-2 deficit in the opening set to defeated Russian Dinara Safina 7-5 6-3.

Safina, the debutant at the Sony Ericsson Championships, let the two-time semifinalist Williams win five straight games to take the first set. Williams had little trouble in closing out the win in the second.

Second seed Safina will play Serena Williams in her next round-robin match, while seventh-seeded Venus Williams will play Olympic champion Elena Dementieva. (photo: Getty Images)