Serena Williams has admitted her surprise at beating Timea Bacsinszky to reach the final of the French Open.

The world number one was struggling with illness throughout this evening's match at Roland Garros, but she was able to battle back from a set down to prevail 4-6 6-3 6-0.

Williams, who will now compete in her third final at the Grand Slam event when she takes on Lucie Safarova, revealed after the match that she has been fighting to overcome a bug for the past week.

"I don't think I've ever been this sick – I didn't expect to win that, and I can't believe I won," she is quoted as saying by The Guardian.

"I got the flu after my third-round match and I haven't been getting better. Hopefully this is the worst and I'll get better. I really thought I was going to lose.

"I thought - if I'm going to lose, I might as well go for winners. I was so tired, and I just fought for every point."

Williams has won the French Open title on two previous occasions.