World No 1 and defending champion Serena Williams lost in the second round of the French Open, beaten 6-2 6-2 by Spain's Garbine Muguruza.
The 17-times grand slam champion never got into her stride against the 20-year-old world No 35.
Top seed Williams had not been knocked out of a major before the fourth round since losing in the first round of the 2012 French Open.
Muguruza, whose powerful groundstrokes and pace around the court proved too hot to handle for Williams, will next face Slovak teenager Anna Schmiedlova, who earlier in the day beat Serena's older sister Venus.
Williams, who made 29 unforced errors and hit only eight winners, said: "I don't think anything worked for me today.
"It was one of those days. You can't be on every day, and, gosh, I hate to be off during a grand slam. It happens. It's not the end of the world.
"I think she played really well and she played really smart. It's great because I'm going to go home and work five times as hard to make sure I never lose again."
Williams' 6-2 6-2 defeat was her worst in terms of scoreline in her 16-year grand slam career and will send shockwaves through the tournament, which lost Australian Open champions Stan Wawrinka and Li Na in the first round.
Like Rafael Nadal on Monday, Williams was sent out to Roland Garros' second court, Suzanne Lenglen.
But, while her fellow defending champion barely broke sweat in beating Robby Ginepri, Williams began badly and got worse.
Despite her 17 Grand Slam titles and huge experience, it is not unusual for Williams to have days where nerves get the better of her and her movement deserts her.
One of the most painful defeats of her career came in Paris two years ago when she lost to Frenchwoman Virginie Razzano in the first round.
That was the moment she turned to coach Patrick Mouratoglou and began one of the most dominant spells of her career.
She has been particularly dominant on clay and last year finally won her second French Open crown 11 years after her first.
Williams was the overwhelming favourite to retain her title but, after winning the first game in cold and heavy conditions, she lost the next five.
Much of the credit must go to 20-year-old Muguruza, who struck the ball superbly and maintained her aggressive approach to ensure there was no way back for Williams.
Muguruza clinched victory on her first match point when Williams netted a backhand.
Maria Sharapova is now the leading favourite to take the 2014 title in Paris.
The Russian battled past Bulgaria's Tsvetana Pironkova 7-5 6-4 to reach the third round. However, the four-time Grand Slam winner, who won Roland Garros in 2012, will have to raise her game after an alarming 27 unforced errors.
The 27-year-old improved her record on clay this year to 14 and 1, after wins in Stuttgart and Madrid left her with 31 titles on the WTA circuit.
The world No 8 now faces Romania's Monica Niculescu or Paula Ormaechea of Argentina for a place in the last 16.
Third seed Agnieszka Radwanska of Poland also went through with a comfortable 6-3 6-4 win over Karolina Pliskova of the Czech Republic.
Radwanska will face Croatia's Ajla Tomljanovic for a place in the last 16.