Elena Rybakina of Kazakhstan pulled off a major surprise in the fourth round of Roland Garros on Sunday, as she upset three-time champion Serena Williams 6-3, 7-5.

"I'm so happy with the match today," Rybakina said in her post-match press conference. "Of course when I went on court I didn't expect anything. I just had a set plan, which we discussed with my coach. I just tried to follow it. It worked out today."

No.21 seed Rybakina was playing in her first Grand Slam round of 16, while 7th-seeded Williams was contesting her 64th -- the first of those coming at 1998 Roland Garros, a full year before 21-year-old Rybakina was born.

However, Rybakina exhibited the form that pulled her to four WTA singles finals and a Top 20 ranking in the first two months of 2020, and she has appeared to rekindle that fire this fortnight, where she has attained back-to-back wins for the first time since the first week of the season.

In the first meeting between the two players, Rybakina's one-hour and 17-minute upset of the 23-time Grand Slam champion Williams came after the Kazakh converted five of her seven break points. She also won nearly 60 percent of points returning the Williams second service.

"When I was small, of course I was watching her matches on TV, so many Grand Slams," Rybakina said. "It's difficult to expect anything, because you watch on TV and it's completely different when you come on court and you feel the power and everything.

"I knew that the serve was going to be difficult for me to return. She's powerful, but I was ready."

Williams' 15 winners were outweighed by 19 unforced errors, as her quest for a historic 24th Grand Slam singles title will have to wait until another attempt at Wimbledon. Williams had been 14-0 against players from Kazakhstan before Rybakina pulled off the upset.

"I got some good matches in here," Williams told the press, after the match. "I did not have the best clay-court season, but it was good to finally get some wins on clay.

"It was definitely close. There is literally a point here, a point there, that could change the whole course of the match. I'm not winning those points. That literally could just change everything."

Rybakina, now into her first Grand Slam quarterfinal, has yet to drop a set during the fortnight. Moreover, two first-time Grand Slam semifinalists and a first-time Grand Slam finalist are guaranteed from the bottom half of the draw, with Rybakina the highest-ranked player remaining in that half.

Rybakina will now face No.31 seed Anastasia Pavlyuchenkova of Russia in the quarterfinals. Pavlyuchenkova followed up her third-round upset of No.3 seed Aryna Sabalenka with another stunner of a higher-seeded Belarusian, eliminating No.15 seed Victoria Azarenka earlier on Sunday.

Pavlyuchenkova is into her seventh career Grand Slam quarterfinal -- she has made it that far in all four Grand Slam events -- but has yet to convert one of those chances into a Grand Slam semifinal appearance.

"[Pavlyuchenkova and I] also play doubles together," Rybakina noted, and the pair will play their round-of-16 doubles match together on Monday. "We are a good team, good atmosphere. But for the [singles] match after [doubles] tomorrow, we come on court to work, and I'm gonna discuss again the tactics and everything with my coach.

"For sure, I have to serve well again because it's my advantage, and then we'll see how it goes."

Rybakina got off to a stunning start, using sturdy groundstrokes to force errors from the long-time former World No.1 and break for a 3-1 lead. However, Williams edged back on serve after a tremendous tussle, converting her third break point of a protracted game to pull within 4-3.

But Rybakina was unbowed by the comeback and slammed a forehand winner to break Williams again for 5-3. Serving for the set, Rybakina surged ahead and held three set points, but each of those times, Williams prevailed. But the American missed a backhand pass long to give Rybakina a fourth chance, and the Kazakh took that one after a miscue wide by Williams.

In the second set, clutch returning by Williams helped her come back from a break down two times, as the players moved to parity at 5-5. However, Rybakina leapt ahead one more time, breaking Williams at love with some key crosscourt shots to reach 6-5.

With Rybakina serving for the match, Williams forced an error from the Kazakh in the first point of the game, but Rybakina settled herself with a subsequent ace, and she eased to a famous victory from there.

"The end goal is always to win the tournaments, and dream, and go of course to win the Grand Slam," Rybakina said. "So every time I step on court, I try not to think against who I play. Just try to do my work, follow the plan which we have.

"If it works well, I'm winning, and if it's not, I'm just learning from the matches. It's all experience for me."