Serena Williams has pulled out injured [GALLO/GETTY]

World number three Serena Williams and fourth-ranked Ana Ivanovic have withdrawn from the WTA Championships in Doha.

Williams, who also pulled out last year after playing just one set, blamed a sore stomach muscle.

Ivanovic, who is suffering from a virus and after losing her first two matches could not qualify for the semi-finals, defaulted her last match against Russian Svetlana Kuznetsova on Friday.

Hotel hurt

Although Serena won just one game in the last two sets of her defeat by sister Venus on Thursday, the American said she only felt the problem develop back at her hotel.

"I felt I didn't serve well, I had a lot of double faults," Serena said.

"Looking back I think maybe that's why I was doing it because I didn't feel it (the injury) at the time.
 
"I think I might have been over-compensating but the moment I got home I did feel pain. When I woke up around 2pm I was still in pain just getting out of bed."

After beating Dinara Safina and losing to Venus, Serena needed to defeat Olympic champion Elena Dementieva on Friday to qualify for the last four.

Maroon Two 

Jankovic lets it slip [GALLO/GETTY]

Zvonareva remained undefeated on Friday, adding a 2-6 6-3 6-4 victory over Jankovic to two previous wins.

In Saturday's semis, Zvonareva meets Dementieva and Jankovic faces Venus.

Ivanovic received medical attention for dizziness in both her matches this week and said her virus had become worse.

"The other night was a very intensive and long match and I started to feel worse," said the Serb.

"Yesterday I didn't feel good and today even worse.

"I tried to hit yesterday. I was on the court for about 10 minutes and as long as I didn't move it was okay.

"But if I tried to hit a little bit harder or to move to the side it was impossible."

Venus rising

Venus Williams defeated her sister 5-7, 6-1, 6-0 to reach the semi-finals on Thursday.

The Williams sisters are 9-9 in career matchups.

Serena defeated Venus in the US Open quarterfinals en route to the title this year, and Venus won their title match at Wimbledon.

The power serve [GALLO/GETTY]

Venus stayed composed throughout the match, using her powerful serve to
frustrate Serena, but refrained from celebrating after winning.

"It wasn't the happiest match," Venus said.

Both sisters played their trademark power tennis in the first set before the rallies turned shorter in the second, with Serena growing increasingly frustrated by her errors.

Venus earned two set points in the second when Serena double-faulted, and
converted the second with a crosscourt backhand that passed her younger sister at the net.