World number one Roger Federer reached the Monte Carlo Masters final when Serbian Novak Djokovic pulled out of their semi-final on Saturday.
The Swiss top seed was leading 6-3, 3-2 when Djokovic complained that he felt dizzy and retired.
Federer will face Rafael Nadal in Sunday's final after the Spaniard beat Russian fourth seed Nikolay Davydenko 6-3, 6-2.
"I didn't feel good for the last three days. I've been waking up with a sore throat," said Djokovic.
"But I thought it's not going to be nothing special. But obviously when you're playing against the number one player of the world, you obviously get a lot of balls back and longer points, and I just couldn't get enough energy back after each point.
"I needed a lot of time. And I just decided not to risk anything. I feel dizziness a little bit in the last three days," he added.
"I don't know. The doctor in the tournament couldn't give me the right diagnosis."
In February, Djokovic retired from a Davis Cup match against Davydenko, complaining of feeling dizzy and struggling for breath, following a bout of influenza.
