An ailing Serena Williams has fought back from a set and service break down to secure a spot on Saturday’s French Open final with a dramatic win over unheralded Swiss Timea Bacsinszky.

A distressed Williams looked close to retiring on several occasions before pulling the semi-final out of the fire with a 4-6 6-3 6-0 victory on a scorching Thursday in Paris.

Chasing a 20th grand slam crown and third title at Roland Garros, the top-ranked Williams will face Lucie Safarova in the championship match after the 13th-seeded Czech beat former winner Ana Ivanovic 7-5 7-5 in the first semi-final.

Williams entered her match with a 23-from-26 record in grand slam semi-finals, her only defeats coming against her sister Venus at Wimbledon in 2000 and to Belgium’s fellow former world No.1s Justine Henin in Paris in 2003 and Kim Clijsters at the 2009 US Open.

But after battling coughing and sneezing fits during her pre-match warm-up, the top seed was on the brink when trailing by a set and 3-2 on Bacsinszky’s serve in the second.

The tournament’s 23rd seed had never progressed beyond the third round in 20 previous grand slam appearances, but was threatening to set up one of the most unexpected of finals in history.

Williams, though, is regarded as one of the fiercest competitors the game has ever known for good reason.

The American dug deep into her reserves to break straight back for 3-3 before reeling off another nine games straight to power into her third final in Paris.

Champion in 2002 and 2013, 33-year-old Williams will become the oldest women’s champion in almost half a century if she raises the trophy again on Saturday.

The final seemed light years away midway through the second set on Thursday.

Williams looked lethargic and unwell from the opening point and it wasn’t long before she was calling for more ice towels on easily the hottest day of the tournament so far.

Looking more and more distressed as the match wore on, the world No.1 ambled to the changeovers before placing the towels under her arm, behind her neck and on her forehead.

Bacsinszky gained the lone break of the opening set in the fifth game with a delightful topspin lob and thunderous backhand crosscourt return winner.

Williams staved off two set points but pulled a forehand wide on the third as the Swiss underdog clinched the first set in 42 minutes.

The 19-times major winner continued to live on the edge in the second set, dropping serve again at 2-2 before stirring to life.

Earlier, Safarova defeated Ivanovic in straight sets to become the first Czech finalist since Hana Mandlikova in 1981.

Safarova recovered from a slow start and inevitable semi-final nerves to advance after one hour and 52 minutes.

Champion in 2008 and runner-up the previous year, Ivanovic led 5-2 early but was on the back foot from the moment she conceded the opening set.

Serving for a place in the final at 5-4, Safarova coughed up three double-faults and dragged a backhand into the net to hand the one-time world No.1 a lifeline.

But the Serb faltered again herself before southpaw Safarova served out the match at the second time of asking to go one better than her run to last year’s Wimbledon semi-finals.

The victory also guarantees Safarova a place in the world’s top 10 for the first time on Monday.