Sloane Stephens, the 2017 United States Open champion, bid toward a second title ended abruptly in the first round on Tuesday night, falling to the qualifier Anna Kalinskaya, 6-3, 6-4.

Stephens, the 11th seed, oscillated between aggressive and counterpunching play, but could not find a consistent rhythm to deter the 20-year-old Kalinskaya, whose steady, powerful attacking from the baseline held up under pressure. Kalinskaya, ranked 127th , hit two fewer winners than Stephens’s total of 20, but offset that by producing 18 fewer unforced errors.

“I was playing one good point, one bad point, one good point,” Stephens said. “The inconsistency doesn’t help me at all. She did a good job just staying there, keeping fighting. She never really let me in. Kudos to her. She executed her game style really well.”