Flu-stricken Serena Williams is savouring a 20th grand slam triumph, saying she can hardly believe she’s won the French Open after dragging herself out of bed to deny Czech underdog Lucie Safarova in a gripping final in Paris.

The world No.1 admitted she feared having to pull out of the final before overcoming nine double-faults and a mid-match meltdown to defeat Safarova 6-3 6-7 (2-7) 6-2 on Saturday.

“I can’t believe I won, but it’s cool,” Williams said after ruling Roland Garros for a third time following victories also in 2002 and 2013.

“It’s been a nightmare 48 hours.”

The oldest winner since 1958, 33-year-old Williams said she “barely made it home” after her dramatic three-set semi-final comeback against Timea Bacsinszky.

“I took a hot shower, I went to bed and then I didn’t leave bed until, like, four five yesterday (afternoon),” she said.

“I was just thinking, I have to go for a walk. This house is making me sick. I have to get out.

“I have to get some air. Went for a walk. Came back. Got worse.

“So I talked to all the doctors here and the physios here. I even told the physio: `I’m not sure I’m going to be able to play because this is just not looking good’.”

But went out and play she did, and the American’s fighting win lifted her above 1920s legend Helen Wills-Moody into outright third place on the women’s all-time grand slam leaderboard.

Only Australian Margaret Smith Court, with 24 slams, and Steffi Graf (22) remain ahead of Williams, who could well match the German’s open-era record haul by the end of the year.

After landing the French Open in such a dire state, the American will be a raging favourite to snare a sixth Wimbledon trophy next month and a seventh title at the US Open in September.

In an extraordinary display of longevity and domination, Safarova was Williams’ 12th grand slam final victim.

Her first scalp was Martina Hingis way back in 1999 in New York.

Since then, Williams has also accounted for older sister Venus – on six occasions – as well as Lindsay Davenport, Maria Sharapova, Jelena Jankovic, Dinara Safina, Justine Henin, Vera Zvonareva, Agnieszka Radwanska, Victoria Azarenka and Caroline Wozniacki in grand slam title matches.

Her only defeats in 24 major finals have come against Venus, Sharapova and Australian Samantha Stosur at the 2011 US Open.

Safarova, who had lost all eight previous encounters with Williams, threatened a huge boilover on Court Philippe Chatrier after stealing the second set and opening up a 2-0 buffer in the decider.

But Williams’ class and vast experience ultimately came to the fore as the top seed rallied back to reel off the last six games to take the thrilling match after two hours and one minute.

“I just couldn’t find any weapon that could stop her,” Safarova said.

“I was trying to mix up the serve, trying to mix up the rhythm, trying to go for risk shots.

“But when she was on – she was just serving amazing and going for the returns, pressuring me right away – it’s just hard to do anything with that.”

There was no suggestion early on that this would be anything but a routine day out for Williams as the all-conquering world No.1 wrapped up the opening set in 31 minutes and charged to a 4-1 lead in the second.

But the top seed twice uncharacteristically dropped serve with double-faults and, even after breaking Safarova for a fourth time, was unable to close out the match at 6-5.

More double-faults and unforced errors flowed in the tiebreaker as Safarova’s relentless counter-punching – and Williams’ jitters – forced the deciding set.

But, not the first time she’s suffered grand slam final nerves, Williams steadied and, after winning one 19-shot rally that featured a desperate left-handed return to consolidate for a 5-2 lead, she finally dismissed Safarova.

She earned 1.8 million euros ($A2.6 million) for her latest grand slam triumph, taking her career on-court earnings to almost $90 million.

Safarova can console herself with a cheque for 900,000 euros ($A1.3 million) and a rise to a career-high world No.7 on Monday.

The first Czech finalist since Hana Mandlikova won the championship in 1981 will also feature in the women’s doubles decider on Sunday with American partner Bethanie Mattek-Sands against Australian Casey Dellacqua and Kazakhstan’s Yaroslava Shvedova.