Defending champion Angelique Kerber's woeful season continued with a first-round loss to 19-year-old Naomi Osaka at the US Open.

The German's 6-3 6-1 defeat makes her only the second defending women's champion to lose her first match after Svetlana Kuznetsova in 2005.

Kerber rose to the top of rankings after winning in both Australia and New York last season but has not claimed a title since nor made a grand slam quarter-final.

Fourth-round appearances at the Australian Open and Wimbledon sandwiched an opening-round defeat at the French Open and her misery was complete here.

It was a very tough draw for Kerber against a player regarded as one of the future stars of the women's game, and Osaka rose to the challenge under the roof on Arthur Ashe.

The Japanese-American is a fearless ball-striker and Kerber had no answer, netting a final forehand to succumb meekly after just 65 minutes.

Kerber, who despite her poor form held the number one spot until Wimbledon, will now fall outside the top 10.

Angelique Kerber disappointed after her US Open exit.