After a season in which she jubilantly found the answers to success, Angelique Kerber somberly has only questions.

The defending U.S. Open champion is gone after a first-round loss Tuesday to Naomi Osaka under the closed roof in Arthur Ashe Stadium. Kerber became the first defending champion to lose in the first round since Svetlana Kuznetsova in 2005, and she went down meekly, 6-3, 6-1.

“It was not my day, completely not my day today,” Kerber said. “Of course, matches are always different than practice, and I just can say that I was practicing a lot in the last few weeks and months.”

But practice hasn’t led to perfect. The 2016 season was the pinnacle of Kerber’s career. She won the Australian Open at the start, her first Grand Slam victory. She closed out the Slam season with her championship at the Open, defeating Karolina Pliskova in the final. Kerber rose to No. 1 in the world rankings, at 28 the oldest player ever to debut there.

Kerber hasn’t won a tournament since that Open victory. Her match record is 25-18 this season. She lost in the final of a small tournament in Mexico, the only final she has reached this year. She has fallen to No. 6 in the rankings and will drop out of the top 10 after the Open.

“I think, of course, this year is a completely different year,” Kerber said. “I played not so well like last year, and I had much less matches last year . . . Gives me maybe also not the confidence when you go out there, you know, you didn’t have too much matches before you came here. I need just matches. I think this is the thing.”

Now she will search for answers apart from a tournament in a city she has grown to adore. “I came here with a good feeling. I was practicing good. I mean, I love this tournament. I love the city. And it’s still like this,” Kerber said. “Of course, it’s not the result I was expecting. But, I mean, I tried my best, and at the end, it was completely not my day.”

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Osaka’s substantial game can suffer from inconsistency, but she kept up the pressure Tuesday with a strong return of serve and a powerful forehand, plus footwork that set up her groundstrokes. The 19-year-old had nice run at the Open last year, reaching the third round before losing to Madison Keys in three tough sets.

Pliskova, who took over Kerber’s No. 1 ranking after Wimbledon, opened with a 6-2, 6-1 win over Magda Linette.