Venus Williams won her second match in two days at the season-ending WTA Championships on Wednesday when she beat Elena Dementieva 6-4, 4-6, 6-3 to leave herself on the brink of the semis with one round-robin match to play.
Russian Vera Zvonareva also remained undefeated after a 6-3, 6-7, 6-4 victory over Serbia's Ana Ivanovic in the white group.
Both Williams and Russia's Dementieva had difficulty settling into the the maroon group match and just one service game was held in the first set.
But the quality of play improved as the match progressed, with many well-contested rallies as each player sought an advantage.
Serving was another matter. Williams struggled with hers throughout the match, three times hitting a pair of double faults and finishing the match with 10 in all.
But it was Dementieva, who in her eight appearances at the season-ending event has won just three of her 18 matches, who crucially double-faulted to give Williams a break for 5-3 in the final set.
Bizarrely, the American then called for a fresh pair of shoes before serving out the match.
Ivanovic, who suffered a spell of dizziness while losing on Tuesday to Jelena Jankovic, suffered another attack before she was beaten by Zvonareva. The Serbian halted play when down 4-1 in the opening set and her blood pressure was checked before she continued.
Although there were occasional moments of genius from both players, the match was mostly an untidy affair with neither able to take control.
Ivanovic needed four set points to level at one set all, and even after Zvonareva built a 3-0 lead in the final set for the loss of only three points she had trouble closing out the match.
The Russian served at 5-3, but on match point Ivanovic made a winning return and then broke serve. The two hour 26-minute battle finished in the next game when Ivanovic netted a forehand on her opponent's third match point.
"I don't like to complain, but ever since I came here I wasn't feeling really well," Ivanovic told reporters. "I got a virus and I had pain in my throat. So from the first day I wasn't feeling so good.
"It was very hard for me to breathe, and I was feeling very dizzy and light headed so it was very tough."
Like Ivanovic, Russia's Dinara Safina has also lost her opening two matches this week, this time surrendering 6-4, 6-1 to Serena Williams.
Although often matching her opponent for aggression, Safina lacked the consistency of Williams in the first set and resilience in the second.
Williams, playing in the same group as her sister, revealed that Barack Obama's US presidential election victory had put a spring in her step when she stepped onto court.
"I did feel a little proud out there tonight," she said after winning her opening match. "It's just a great time to be black right now in the United States. Everyone's proud. I did feel like my chest stuck out a little bit."