Garbine Muguruza has registered the biggest win of her still-young professional career, sending shockwaves through the grounds at the French Open.

In the biggest victory of her career to date, the 20-year-old Venezuelan-Spanish tennis phenom outplayed Serena Williams, the defending French Open champion and World No. 1, at Roland Garros.

Williams, a 17-time major winner, lost to  Muguruza 6-2, 6-2, in her earliest exit at a major tournament since falling in the first round at Roland Garros two years ago. It’s also her most-lopsided loss ever at a Grand Slam.

Muguruza was nerveless serving out the biggest win of her life, clinching victory on her first match point when Williams netted a backhand.

“It was one of those days. You can’t be on every day, and, gosh, I hate to be off during a Grand Slam,” said Williams. “It happens, you know. It’s not the end of the world. It is what it is.”

Williams struggled from the start against Muguruza and finished the match with only eight winners and 29 unforced errors.

“Just nothing really worked,” Serena said. “I don’t know anything that actually worked.”

The up-and-comer dictated most of the points, stayed aggressive and often smacked her groundstrokes down the middle of the court to avoid giving Williams angles — an approach Muguruza reminded herself to follow while preparing for the match in the locker room. Muguruza hit 12 winners against 18 unforced errors. During their postmatch handshake, Muguruza said Williams told her that if she continues to play this way, she can win the tournament. “I will try, I will try,” Muguruza responded.

Muguruza was playing at the French Open for only the second time in her career. She lost in the second round last year, but reached the fourth round at the Australian Open in January.

In their only other meeting, Williams crushed Muguruza 6-2, 6-0 at the 2013 Australian Open. “I was horrible because I was so nervous,” Muguruza said. But since then she has climbed from No. 112 into the top 40. In January, she won the Hobart International as a qualifier for her first title, then reached the fourth round in Melbourne, the final of the Brasil Tennis Cup and the semifinals of the Grand Prix SAR.

“I think now I believe more in me,” she said. “I’m more calm, because I know that I can play good.”

Still, Muguruza entered the match as a heavy underdog, having gone 0-5 in her career against top-five players and 3-8 against the top 10. In her two most recent tournaments, she had lost to No. 19 Sam Stosur in the second round of the Madrid Open and to No. 61 Francesca Schiavone in the second round of the Italian Open.

Muguruza will next face Slovak teenager Anna Schmiedlova, who earlier in the day beat Serena’s older sister Venus WIlliams.