“I’m kind of really glad that’s over,” Osaka said afterward.

The last defending U.S. Open women’s champion to lose in the first round was Angelique Kerber, in 2017. She lost to a player ranked 45th in the world, a 19-year-old by the name of Naomi Osaka.

Osaka attributed her nerves on Tuesday to the fact that it was the first time she had walked on court at a Grand Slam tournament as a reigning champion, and she said she knew it would have been disastrous to lose the opening foray into defending her title.

She compared it to her defense of a title in Indian Wells, Calif., where she won in 2018 and lost in the round of 16 this year. But the pressure she exerted on herself Tuesday seemed to be double that.

“The amount of nerves that I felt today was completely different than in California,” Osaka said, adding that the feeling never went away.