Ivanovic mixed some stunning strokes with some unforced errors
Fourth seed Ana Ivanovic lost for the second time in two days as Russian Vera Zvonareva triumphed 6-3 6-7 (5-7) 6-4 in the Tour Championships in Doha.
The Serb, 21 on Thursday, made a slow start and had to be assessed by a doctor after having dizzy spells.
She won the second set after wasting three set points but lost the match.
In Group B, Venus Williams won again, beating Elena Dementieva 6-4 4-6 6-3 and next meets sister Serena, who saw off Dinara Safina 6-4 6-1.
Zvonareva launched a furious outburst as an over-rule went against her in the first game of the decider, but she kept her focus.
Ivanovic played some delightful strokes but was plagued by inconsistency again and now has little chance of progressing, with only two players from each group qualifying for the semi-finals.
She squandered a chance for a commanding lead in the second-set tie-break when Zvonareva did well to return two smashes in a thrilling rally.
But a magnificent cross-court pass created the fourth set point she needed to take the match to a final set.
Zvonareva's frustration with the umpire had a positive impact on her performance, however, and she raced into a 3-0 lead.
The determined eighth seed held on for victory and said: "I managed to win a very tough match against a tough opponent."
Ivanovic said: "I have had a virus, but I don't know why my blood pressure was taken - it frightened me and made me feel worse.
"It was very hard for me to breathe, and I was struggling. I was feeling very dizzy and light headed. So it was very tough. I couldn't eat much for the last couple of days and with the lack of energy, it was a little bit hard.
"It was very disappointing, but I still gave 100% in this match, and she was the better player. I need a miracle to qualify now."
Venus maintained her fine start with a gruelling victory over Olympic champion Elena Dementieva.
Williams, who beat another Russian, world number two Safina, on Tuesday, fought back from 0-2 down in the deciding set to secure the win in two hours 21 minutes.
The American had problems with her serve, however, with a total of 10 double faults, and three times sent down pairs of double faults.
"It was just really competitive out there," the 28-year-old said.
The Wimbledon champion may be able to qualify for the last four even if she loses her final group match against her younger sister.
Safina had no better luck against Serena on Wednesday as the Russian lost in straight sets for the second day running.