Serena Williams French Open 2015 Semifinal
Serena Williams was visibly unwell, but still found a way to get past Timea Bacsinszky in the semifinal of the French OpenReuters

An ailing Serena Williams stayed on course for a 20th Grand Slam singles title as she came back from the brink to reach the final of the French Open with a 4-,6 6-3, 6-0 win against Swiss power hitter Timea Bacsinszky.

The American, who looked unwell throughout the match, putting iced towels around her neck and against her forehead at changeovers, fell a set and a break down as the 23rd-seeded Bacsinszky peppered the court with backhand winners.

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But Williams, who won Roland Garros in 2002 and 2013, dug deep to break back and Bacsinszky cracked, losing 10 games in a row.

Williams closed it out when Bacsinszky sent a backhand long and the world number one will now face Czech 13th seed Lucie Safarova on Saturday.

"I have tried everything. I thought if I lose I must fight. I have tried and tried, I don't know where I found the energy," Williams told a courtside interviewer in French before a coughing fit ended the conversation.

"I have been feeling unwell for a few days," she later added in a statement, explaining she had to see the tournament doctor after the match.

Last year, Williams pulled out of a doubles match with her sister Venus at Wimbledon after appearing disoriented and weak on court when she was suffering from a viral illness.

There was no quitting this time, however.

Bacsinszky had promised not to be intimidated and she was true to her word at the start of the match.

The 25-year-old broke for 3-2 with a quickfire service return winner, leaving a subdued Williams to walk back to her chair at a snail's pace.

Williams had her only break point of the set in the 10th game, but the Swiss saw it off with a trademark backhand winner.

Bacsinszky then had set points, but fired long on the first and Williams saved the second with a backhand winner down the line.

On the third, the American had her opponent on the ropes, but with the court wide open, she fired a forehand wide.

Bacsinszky broke in the fifth game of the second set and Williams fought back tears at the changeover.

She came back fighting, however, to break back for 3-3, held serve and broke again for 5-3.

She served it out, clenched her first and twirled in celebration.

This time it was Bacsinszky who looked disoriented and she never recovered as Williams motored away.