Never before had the 13-times Grand-Slam champion, one of the pre-tournament favourites for the title, lost in the first round of a major but on Tuesday she paid the price for a performance she summed up as full of "errors".
The fifth seed was two points from victory at 5-1 up in the second set tiebreak but her game inexplicably collapsed as she lost six points in a row to lose the set.
The American appeared close to tears during the changeover at the end of the set and struggled to shake off her near-miss in the decider as she allowed her 111th ranked opponent to romp into a 5-1 lead.
But Williams, who believes herself to be the ultimate fighter in the sport, refused to throw in the towel and scraped back to 5-3 as Razzano struggled to deliver the knockout punch.
Just when it seemed as if Razzano would join a long list of players who choke at their home Grand Slam event, the Frenchwoman played her heart out.
A cramping Razzano held her nerve in a jittery, action-packed ninth game of the final set, which lasted 25 minutes, stretched to 12 deuces and featured four break points.
There was even a moment of controversy as Razzano was penalised by the umpire for letting out a loud squeal mid-point, leading to a loud chorus of boos from the fans.
The Frenchwoman took it all in her stride before finally winning the contest on her eighth match point.
Such was the drama of the match that Razzano was even made to wait before she could celebrate her glorious victory as the umpire had to climb down from her chair to confirm Williams's backhand was long.
Razzano, who lost her fiance to a brain tumour a year ago, jumped up in joy after surviving three hours and three minutes of topsy-turvy tennis that delighted the hollering crowd in Court Philippe Chatrier.
While Williams swiftly packed her bags as she digested her earliest Slam exit in 47 appearances, Razzano celebrated her greatest triumph.
"I didn't think too much. I know I have unlimited resources. I dug very deep and I knew nothing was lost, even when I had cramps (in the last game), even when I was not feeling well," she told a courtside interviewer.
"It's the most beautiful victory in my career.
Williams did not mince her words as she summed up her day.
"I made so many errors today which is not the game that I have been playing in the past. There is no excuse," Williams admitted.
World number 111 Razzano next faces Arantxa Rus of Holland for a place in the third round.
Russian second seed Sharapova was her usual sulky, aggressive self, peppering a sunsoaked Court Suzanne Lenglen with winners to crush the world number 78 in 48 minutes.
Sharapova, who has a chance of taking over from Victoria Azarenka as world number one depending of the Belarusian's run in Paris, blew kisses to the crowd after wrapping it up when Cadantu sent a backhand long on the first match point.
The Russian, who needs the French crown to complete her Grand Slam title collection, will next face Japan's Ayumi Morita.
"I feel like with every year I have improved and I enjoy it much more," the Russian former world number one said. "I feel like I'm moving a lot better than I did in years previously, which has helped me a lot in the recovery process within the point."
Czech Republic fourth seed
The Wimbledon champion needed just 54 minutes on a sundrenched Court Suzanne Lenglen to subdue her 16-year-old opponent.
World number 332 Barty broke Kvitova's serve in the first game of the second set but the Czech had simply too much pace.
Kvitova will next meet either France's Pauline Parmentier or Poland's Urszula Radwanska.
The Italian, 2010 champion at Roland Garros and runner-up last year, was never severely tested by Date-Krumm, the third-oldest woman to play at the French in the professional era, at the age of 41.
Date-Krumm, who made her Roland Garros debut in 1989 and reached the last four in 1995, saved two breakpoints at 0-2 down in the second set but surrendered two more service games despite some occasional impressive shots.
Schiavone, seeded 14th, said she had felt nervous about stepping out on the Philippe Chatrier court again after losing last year's final to Li Na.
"Before going out I was really very tense because to play on the central court, to feel all you can feel on such a big court and to be here with you hopes and dreams is very difficult. But as soon as I came on, I started to breathe and to move," she said.
Schiavone paid tribute to Date-Krumm, saying: "I think Kimiko is a really fantastic athlete. It doesn't matter if she wins many matches or not, I think she is an example for everybody - she works a lot, she respects the other players.
"I think we have to learn many things from her," added Schiavone. "For me it's great to play against her."
The Italian will now play