Maria Sharapova was left frustrated by her inconsistency after making a fourth-round exit at the US Open on Monday.
The Russian's perfect record in night matches on Arthur Ashe Stadium was ended in a surprise 6-4 6-3 defeat to Carla Suarez Navarro.
Since returning from her drugs ban last year, Sharapova has made just one quarter-final in five grand slams.
Asked what the most disappointing aspect of her game was, the five-time major champion said: "I think not being able to consistently put up the level from one match to another, stringing obviously in a span of hopefully a total of seven matches during a grand slam.
"It's just a little too up and down."
Suarez Navarro celebrated her 30th birthday in style, the straight-sets win ending Sharapova's streak of 22 consecutive victories in night matches on Arthur Ashe Stadium.
The seven-time grand slam quarter-finalist, who faces Madison Keys in the last eight, said she was aware of Sharapova's record.
"I know that she never lost on night session because last night I saw her match a little bit, and they put it on the TV. But, well, nothing changes for me about these things," Suarez Navarro said.
"But Maria, she's a really good player. I mean, she's one of the best. Well, I'm happy to beat her."