Kerber states, it's tougher to stay on top

Angelique Kerber has been given a brutal reminder this season that if becoming world number one is hard, staying at the top is even more difficult.

The defending US Open champion was knocked out in the first round at Flushing Meadows on Tuesday when Japan's Naomi Osaka steamrollered her 6-3, 6-1 on the Arthur Ashe Stadium where she lifted the trophy 12 months ago.

German Kerber has not won a title since and will now drop out of the world's top 10.

"It's a new situation. Of course, you have always the goal for years to go there and to reach the top, but then if you are there, you actually don't know what to expect," Kerber, who became world number one after wining the US Open last year, told a news conference.

"You have to get used to everything. You have to schedule your day, plan completely different and it's for sure tougher to stay on top."

She reached the final at the year-ending WTA finals, losing to Dominika Cibulkova, but relinquished the top ranking after being knocked out in the Australian Open fourth round.

She reclaimed it and lost it several times but there were serious concerns over her form going into the US Open and Osaka was merciless in exposing the her weaknesses.

"After Wimbledon I was actually confident, because I was practising good. I knew that everything was going the right way, but I was a little bit injured and it's still not 100 percent with my elbow," the sixth seed said.

"I'm always trying my best and also my team is trying the best, and I think this was also during the whole year I always had some small things that (prevented me from) playing my best.

"I will try to forget the match as soon as possible and look forward again."