Kerber's poor run of form hit a new low when she was knocked out of the US Open by 19-year-old Naomi Osaka on Tuesday.

The German, seeded sixth, lost 6-3, 6-1 as Osaka produced an electrifying performance to record her first victory over a top 10 ranked player and underline her huge potential.

The German has not won a title since her success at Flushing Meadows a year ago and has lost three of the last four matches she has played since Wimbledon in July.

In losing to the world No. 45, Kerber becomes the first defending champion to be eliminated in the first round of the US Open.

'Tough at the top'

"I think because it's a new situation," said the 29-year-old former No. 1 told reporters when asked why it had been so hard for her to cement her place on top of the rankings.

"Of course, you have always the goal for years of reaching the top, but then if you are there, you actually don't know what to expect, and then it's just the process." 

Kerber added: "You have to get used to everything. You have to schedule your day, plan completely differently and it's for sure tougher staying on top."

With second seed Simona Halep's loss to Maria Sharapova on Monday night and seventh-seeded Johanna Konta also falling, three of the top seven seeds on the women's side were gone by midday on Tuesday - with Serena Williams not playing because she is pregnant.

Rain fails to halt Pliskova

Top seed Karolina Pliskova started off her tournament with a win, dispatching Poland's Magda Linette 6-2, 6-1 on a day when rain interrupted play.

Pliskova, who earned the top ranking just over a week ago with a Cincinnati final against Garbine Muguruza, was playing her first match as a Grand Slam top seed.

The match had to be halted briefly when rain stopped play on the outside courts, but play resumed once the new Arthur Ashe Stadium roof had been closed.

The Czech finished with eight aces and 29 winners, with the seed needing to lift her game after a slow start against the world No. 72.

Nadal breezes through

In the men's draw, top seed Rafael Nadal had no problems as he cruised to a straight sets win over Dusan Lajovic 7-6(6), 6-2, 6-2.

After the Serbian had pushed Nadal to a first-set tie break it became a routine victory for the Spaniard.

The two-time US Open champion is to face either Taro Daniel of Japan or Tommy Paul of the US for a place in the last 32.

Nadal's potential semifinal opponent, Swiss third seed Roger Federer, was pushed to five sets in the opening round of the tournament for the first time in 17 years before he overcame American Frances Tiafoe 4-6, 6-2, 6-1, 1-6, 6-4.