Story highlights Naomi Osaka upsets No. 6 seed Angelique Kerber in straight sets

Kerber was the 2016 US Open champion and a former world No. 1

(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.

UPSET ALERT:@Naomi_Osaka_ takes out defending #USOpen champion, 6th seed Angie Kerber.



Did you see that one coming?! pic.twitter.com/dtCQifGR3L — US Open Tennis (@usopen) August 29, 2017

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.

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