Angelique Kerber will drop out of the world's top 10 following the US Open after her title defence at Flushing Meadows was ended at the earliest opportunity by an inspired Naomi Osaka.

Japanese teenager Osaka produced a superb display at Arthur Ashe Stadium to thrash Kerber 6-3 6-1 in an hour and four minutes.

The sixth seed held her own initially as the first six games were shared, but Osaka won nine of the next 10 with her power too much for Kerber to handle.

After winning two grand slams in 2016, in addition to reaching the final of Wimbledon, Kerber has endured a miserable season. She also exited the French Open at the first hurdle and has not made a slam quarter-final in 2017.

The German was still the world number one as recently as July, but will slip significantly from her current ranking of sixth after the US Open is completed, having become only the second defending women's champion to lose in round one. Svetlana Kuznetsova suffered the same fate 12 years ago.

Osaka, who hit 22 winners on Tuesday, can look forward to a second-round tie in New York against either Denisa Allertova or Rebecca Peterson.