(CNN) The women's US Open defending singles champ is out.
Angelique Kerber, who at the time vaulted to No. 1 in the world after winning the US Open last year, was knocked out Tuesday afternoon after just one round, losing to 19-year-old Naomi Osaka of Japan in straight sets.
Osaka, ranked No. 45 in the world, won 6-3, 6-1. This was their first career meeting.
It's the second time the defending US Open women's champion has lost in the first round the following year. Svetlana Kuznetsova was the other, in 2005.
Osaka had 22 winners, 17 unforced errors and saved all four break points.
"I really had to concentrate to play (Kerber) because she's really good and she gets (to) everything," Osaka told ESPN on court after the win.
A year ago at Flushing Meadows in New York, Kerber beat Karolina Pliskova in a thrilling final, claiming her second grand slam title of 2016. But it's been a struggle for the the German since. Kerber hasn't won a title since she won in New York in 2016.
Kerber, seeded sixth, is the latest of the women's top seeds to fall in the first round this week. No. 2 Simona Halep lost in an instant classic to wild card Maria Sharapova on Monday night, while Great Britain's Johanna Konta, seeded seventh, lost to Aleksandra Krunic of Serbia.
Osaka will face the winner of the match between Denisa Allertova of the Czech Republic or Rebecca Peterson of Sweden in the second round. Last year, Osaka lost to American Madison Keys in the third round.
"I played good today, so I just hope to carry that on to my next match," Osaka said.