PARIS -- Serena Williams' quest for a 24th Grand Slam title ended with her earliest loss at a major tournament in five years.

Williams was outplayed in the third round of the French Open by 20-year-old American Sofia Kenin, who used clean, deep groundstrokes to put together the 6-2, 7-5 upset Saturday.

"She played really well," Williams said. "I feel like she, in that first set in particular, hit pretty much inches from the line. I hadn't played anyone like that in a long time. ... She just played, literally, unbelievable. She really went out there today and did great."

The last time Williams was eliminated this quickly at a major came in 2014, when she lost in the second round at Roland Garros to Garbine Muguruza and in the third round at Wimbledon to Alize Cornet.

"Serena is such a tough player. I'm still trying to process what just happened," Kenin said, about an hour after the match ended with her covering her face with both hands. "She's a true champion and an inspiration."

Serena's 2019 Season Australian Open: Held 5-1 lead in third set and squandered four match points against Karolina Pliskova in quarterfinals; lost 7-5 in the deciding set.

Indian Wells: Retired in second set of third-round match vs Garbine Muguruza (viral illness).

Miami Open: Withdrew before third-round match (knee injury).

Rome: Withdrew ahead of second-round match with sister Venus (knee injury).

French Open: Lost in third round to American Sofia Kenin; 1st time losing in 1st week of a major since 2014 Wimbledon (3rd round).

Since those early-for-her defeats, Williams had won six of the 14 majors she entered to surpass Steffi Graf's professional-era record of 22 Grand Slam singles championships. With 23, Williams stands one away from Margaret Court's mark for the most in tennis history; Court played in both the professional and amateur eras.

Williams, who is 37, sat out four Slams in 2017-18 while she was off the tour to have a baby. Her first major tournament back was last year's French Open, where she withdrew before a fourth-round match because of a chest muscle injury. She went on to reach the finals of Wimbledon and the US Open before wasting match points during a quarterfinal loss at the Australian Open.

Williams came to Paris having played only four matches since then; she withdrew from two tournaments because of an injured left knee and from another because of illness.

"I am glad I came," Williams said. "I love the city. I love the tournament. I really wanted to be here. I'm glad I came. But it's just been a really grueling season for me."

Said her coach, Patrick Mouratoglou: "All I know is, she was not ready. And it was obvious, I think."

Williams struggled through her opening match at the French Open, which she has won three times, and again against the 35th-ranked Kenin, who never before had made it to the round of 16 at a major.

But Kenin played quite well, never showing a trace of nerves. It was Williams whose strokes were off-target: Her 34 unforced errors were twice as many as Kenin's total. And Kenin broke Williams four times, while only ceding one of her own service games despite Williams having six break point opportunities.

Kenin was born in Moscow and is fluent in Russian. Her family moved to New York when she was a baby, and she now is based in Florida.

"I'm proud to be an American," said Kenin, who wore a blue U.S. Fed Cup cap to her news conference. "I think it's great we moved to America for a better life for me."

She next will face Ashleigh Barty in the round of 16.

It was only the third tour-level loss by Williams to an American player younger than her in 42 career matches at any tournament. Kenin joined Sloane Stephens (2013) and Madison Brengle (2017) in that club.

It was the second significant surprise in a matter of hours: Earlier in the day, No. 1 seed Naomi Osaka was eliminated 6-4, 6-2 by 42nd-ranked Katerina Siniakova of the Czech Republic to end Osaka's 16-match Grand Slam winning streak.

ESPN Stats & Information and The Associated Press contributed to this report.