Doha: Dizzy, drained and desperate for breath, Jelena Jankovic dug deep.
Trailing 4-2 to Vera Zvonareva in the first set of her opening match at the WTA Championships on Tuesday, the sickly Serb knew she had to hold serve or kiss the match goodbye.
Zvonareva was running the legs off her, dragging her from corner to corner and back again in an engrossing seventh game that Jankovic could not afford to lose.
Defiant, the Serb somehow held serve but exhausted from the effort she lost the next eight games and went down 6-3, 6-0.
"I had difficulty breathing during the match, felt dizzy and tingling, and felt like I didn`t have any power," said Jankovic, who was unable to attend a post-match news conference.
"I finished the match but I had a really hard time. There were moments I felt I was going to give up but I was able to push through."
A shadow of the athletic counter-puncher who rose to world number one in 2008, Jankovic has been laid low by a virus that saw her exit the last two tournaments in the second round.
"I am still suffering from an illness that affected me last week and I had medical treatment after the match today when I received an IV," she said in a statement.
The punishing round-robin format of the group stage in the $4.55 million tournament offers Jankovic no respite, and she will be plunged straight back into the action on Wednesday against Kim Clijsters in the White Group.
Zvonareva, who could claim the year-end number one ranking from Caroline Wozniacki, faces Victoria Azarenka.