Jelena Jankovic reached the semi-finals of the Sony Ericsson Open after world number 11 Elena Dementieva retired with a back injury on Wednesday.

Jankovic was suffering badly from a cold but did not have to stay on court too long as Dementieva quit midway through the second set.

Jankovic will play world number 20 Vera Zvonareva after the Russian earned a 7-5, 6-4 win over Dinara Safina. Defending champion Serena Williams [Images] will take on Svetlana Kuznetsova in the other semi-final.

Serbian Jankovic was leading 6-1, 3-1 when Dementieva, who had her injury treated on court earlier in the match, decided she could not carry on.

"I'm fighting. I'm sick," Jankovic told reporters. "I cannot play, I cannot breathe on the court. I don't want to go on to the court. But I'm hanging in there and trying to be strong.

"I'm more focused than usual because I know that I'm not 100 percent with my health. I'm coughing a lot. My nose is running."

Jankovic, who earlier in the tournament saved five match points against Sweden's Sofia Arvidsson in the second round, said she had managed to unsettle her opponent.

"I was dictating the points, I didn't let her play her game," the world number three said.

"But, unfortunately, she stopped the match in the middle of the second set, and there's nothing I could do."

Dementieva struggled from the outset, racking up 26 unforced errors and was broken three times before retiring.

"I couldn't play because I felt like it was just getting worse, the back problem. It just started yesterday," the Russian said.

"I tried to do some treatment, to get some massage to relieve the pain, but it didn't help me a lot."

Zvonareva said she had to battle hard against the taller Safina.

"It's always tough to play against Dinara, she's a great player," Zvonareva said. "She has a good serve. So I had to hang in there, had to fight for every point, and I think I did it well and I was able to win key points."

• Complete tennis coverage
• All you want to know about Sania Mirza