Force India's Sergio Perez has revealed he has been fighting a virus over the course of this week and is bracing himself for a "massive challenge" on Sunday.
Perez knew something was wrong when he was unable to keep pace with his trainer during a run on Thursday and his condition has worsened throughout the weekend.
"I'm not so good man, I've had a virus," Perez told Autosport after qualifying ninth.
"On Thursday morning I went for a run and I was not at my normal level. I was struggling to keep up with my trainer.
"I went into my room, took a shower and then was freezing. I had my jacket on, and for the whole day I was really struggling.
"I progressed a lot on Friday but in the car, I struggled to breathe. It really took me down.
"Today, it definitely hurt me in Q3. There was nothing wrong with the car or tyres, I just couldn't repeat the lap I did in Q2.
"If I could have improved that I would have been starting P6, but I was struggling for it already in qualifying."
Looking toward to the race, Perez is preparing himself for a big battle ahead.
He added: "Tomorrow is going to be a massive challenge.
"Already in qualifying I was struggling a bit. I will try to hydrate myself as best as I can.
"It is obviously not the place where you want to be sick, so it is going to be quite a race for me."