With a mistake-filled performance, Sharapova lost a night match at Flushing Meadows for the first time in her lengthy career, beaten 6-4, 6-3 by No. 30 seed Carla Suarez Navarro of Spain on Monday.
"Just a little too up-and-down," is the way Sharapova described her performance.
It's the third consecutive appearance that ended one step short of the quarterfinals for the 31-year-old Sharapova, who had been 22-0 under the lights at Arthur Ashe Stadium.
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, who turned 30 on Monday, will be at that stage for the second time at the U.S. Open, five years after her other run to that round.
"A really complete performance," is the way she described her play.
On Wednesday, Suarez Navarro and her smooth one-handed backhand will take on 2017 runner-up Madison Keys of the United States. Keys advanced by beating No. 29 Dominika Cibulkova 6-1, 6-3.
"I have to be ready," Suarez Navarro said about facing Keys, "and I think I am ready."
Suarez Navarro often 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 repeatedly 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 U.S. Open — in 2012, when she lost in the semifinals. Since then, the best she's 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."