Doha, Qatar - Serena Williams secured a perfect group record Thursday at the Sony Ericsson Championships after a 6-2, 6-4 defeat of Elena Dementieva. Williams, who wrapped up the world number-one ranking a day earlier when rival Dinara Safina was forced out of the year-end event with back pain, added her Maroon group victory to those she scored earlier over Svetlana Kuznetsova and sister Venus.
"I'm really happy to be in the semi-final. I have never done well in round-robin play," Serena said.
US Open finalist Caroline Wozniacki came through a gruelling, physical struggle against substitute Vera Zvonareva, which was marked by fitness issues.
The Danish winner produced a fighting 6-0, 6-7, 6-4 against the Russian, who substituted for her compatriot Safina.
"I have absolutely no idea how I pulled it through, but I'm very happy about it," said Wozniacki.
"(In the final set) from 3-1 I got the cramp in my leg. From there, it just got worse and worse. I'm going to do everything tonight to recover to be ready for tomorrow."
Final standings for the weekend semi-finals will be determined after Friday's last group matches, with Wozniacki in a secure position pending her recovery.
Williams ended the group stage a perfect 3-0 to reach the last four. She converted on her third match point after a first-set recovery, which resulted in seven straight winning games.
World number four Wozniacki suffered with cramping as she played her sixth hour this week. Zvonareva was treated for a second-set nosebleed in the two-hour, 48-minute struggle in the Gulf.
Wozniacki nearly lost her form when the cramps sent her to the floor late in the match. But the teenager soldiered on to earn a third match point and ultimately claim the tense contest.
