NEW YORK -- Carla Suarez Navarro ended Maria Sharapova's perfect record in night matches at the US Open, reaching the quarterfinals with a 6-4, 6-3 victory Monday on her 30th birthday.
The No. 30 seed from Spain will face 2017 runner-up Madison Keys in a bid for her first Grand Slam semifinal.
Sharapova, seeded 22nd, had been 22-0 under the lights in Arthur Ashe Stadium. The 2006 champion has lost in the fourth round of her past three appearances.
"Just a little too up-and-down," is the way Sharapova described her performance Monday.
Sharapova collected the trophy in New York at age 19 in 2006 and owns a total of five Grand Slam titles, but the Russian was far shakier during this match than Suarez Navarro, who never has made it past the quarterfinals at a major.
The Spaniard will be at that stage for the second time at the US Open, five years after her other run to that round.
"A really complete performance," is the way she described her play.
Suarez Navarro let the 22nd-seeded Sharapova create her own problems.
Sharapova had all sorts of trouble serving, repeatedly catching wayward ball tosses and committing eight double faults. She was broken in six of her 10 service games.
During lengthy exchanges from the baseline, Sharapova often blinked first, although a couple of times the righty managed to switch her racket to her left hand for a desperation shot to extend a point.
While both women finished with 15 winners, Sharapova had nearly twice as many unforced errors as Suarez Navarro, 38-20.
"I didn't take care of the chances that I had. By 'chances,' I mean the balls that were a little bit shorter. I hesitated to move forward," Sharapova said. "The balls where I did attack, I made unforced errors, especially on that inside-out forehand today."
Since her championship, Sharapova has only once made it to the quarterfinals at the US Open -- in 2012, when she lost in the semifinals. Since then, the best she has done are fourth-round exits in 2014, 2017 and 2018.
After this latest loss, Sharapova was asked whether she envisions herself getting back to her best in the future.
"First of all, if I didn't have the belief to keep doing this and to keep having the motivation and the grind of doing this every day in order to get myself in these positions, I don't think I would be here. I think I've done plenty in my career, established a lot for myself personally, professionally," she replied.
"The belief is not something that I'm eager to show everybody else," Sharapova continued. "The belief matters most when it's internal and when you have a passion for something. If you don't, it's your choice to not continue that, not for anyone else to tell you so."
Keys returned to the US Open quarterfinals with a 6-1, 6-3 victory over No. 35 seed Dominika Cibulkova of Slovakia.
Keys was a finalist here a year ago and lost to Sloane Stephens. She lost to Stephens again this year in the French Open semis.
Keys has reached at least the quarterfinals in three of the four Grand Slams this year. She had six aces and no double faults Monday in perhaps her best performance of the tournament.
Lesia Tsurenko reached her first Grand Slam quarterfinal by outlasting teenager Marketa Vondrousova 6-7 (3), 7-5, 6-2 after feeling ill earlier in the match.
Tsurenko struggled in the heat. She took long pauses between points in the first-set tiebreaker, then left the court for treatment after it. She still looked weary to start the second, leaning on her racket between points while falling behind 2-0. She said afterward she felt dizzy and was "just asking nature, I don't know, the god, to move the shade."
She was able to play through the pain and eventually won the match in 2 hours, 32 minutes.
The Ukrainian got some help from her opponent. She hit only 17 winners, but the 19-year-old Vondrousova made 90 errors -- 73 unforced.
Earlier Monday, Naomi Osaka became the first Japanese woman to reach the quarterfinals of a Grand Slam in 14 years, edging Aryna Sabalenka 6-3, 2-6, 6-4 in a matchup of 20-year-olds.
The No. 20 seed had to work much harder than she had so far in the tournament, where she had won 22 consecutive games leading into Sunday's match.
She finally pulled it out when the No. 26-seeded Sabalenka double-faulted on match point, tossing her racket to the court in frustration.
The last Japanese woman to reach the final eight in a Grand Slam had been Shinobu Asagoe at the 2004 US Open.
The Associated Press contributed to this report.