Kerber 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. Osaka's win was the biggest for a Japanese player in a major tournament since Akiko Morigami stunned then World No.3 Nadia Petrova at Roland Garros in 2006.
She almost upended Keys in the U.S. Open's third round previous year, leading 5-1 in the third set before losing in a tiebreaker, a collapse on her mind in the latter stages Tuesday.
It was Osaka's first victory over a top-10 opponent after she was oh-so-close to beating a top player here a year ago, blowing a 5-1 lead in the third set and losing to Madison Keys in the third round.
Angelique Kerber will drop out of the world's top 10 following the U.S. Open after her title defense at Flushing Meadows was ended at the earliest opportunity by an inspired Naomi Osaka. "So that helped me focus and concentrate, which I needed as Angelique gets everything back". Playing in the U.S. Open main draw for a third time, Kenin entered the tournament as a wildcard after winning the U.S. Open Wild Card Challenge.
"I'm still the same player and the same person, so I think it's just-yeah, I think it's just the matches and how I played last year from the beginning and how I'm struggling this year", Kerber said of the contrast between her 2016 and 2017 seasons. Her match against No9 seed Madison Keys reduced her to tears, though, as she gave up a 5-1 lead in the final set.
Osaka also converted four break points and won 63.2 percent of Kerber's second serves-again, a sign of her dominance.
Osaka moves on to play the victor of Rebecca Peterson of Sweden and Czech Republic's Denisa Allertova, whose match was postponed due to rain on Tuesday.
With torrential rain drenching the outside courts - allowing just three women's matches to finish by mid-afternoon - Rafael Nadal and Roger Federer at least had the comfort of knowing that they would begin under the roof.
"I think because it's a new situation", said the 29-year-old when asked why it was so hard for her to cement her place on top of the rankings. "You have to schedule your day, plan completely different and it's for sure tougher to stay on top".
Juan Martin del Potro, the 2009 champion in NY, starts against Switzerland's Henri Laaksonen.