No US Open defending champion had lost in the first round the following year since Svetlana Kuznetsova in 2005, but Kerber's service woes deepened in the eighth game of the set, offering up her fourth and fifth double faults en route to suffering the first break of the match.
In turn, Osaka has earned a victory that could propel her into the next tier of WTA players.
Talk about an understatement.
Osaka grew in confidence from that point on and completely blew Kerber away to secure the biggest win of her career.
Her follow-up has been quite a flop. She needed to make it to at least the round of 16 to stay in the top 10 but couldn't get close. Last year, Osaka lost to American Madison Keys in the third round. "But at the end, I know that I'm strong and I know that I will come back stronger, for sure".
Showers showed up before noon Tuesday and led to the postponements of dozens of matches.
No. 23 Barbora Strycova, Pliskova's Czech countrymate, also advanced with a 6-1, 6-3 victory over Japan's Misaki Doi, while No. 28 Lesia Tsurenko of Ukraine was upset by Yanina Wickmayer of Belgium, 6-3, 6-1.
The Czech defeated Poland's Magda Linette 6-2 6-1, and was relieved to safely negotiate the first hurdle. "I just barely returned the serve", said Osaka, whose father is Haitian and mother is Japanese.
Man with knife shot dead after attacking soldiers in Brussels
About an hour later, two police officers were attacked outside Buckingham Palace in London, police said. Images showed police have sealed off a main street near the main Grand Place tourist attraction.
Osaka lost to Madison Keys previous year but had led 5-1 in the final set before her nerve failed.
Rafael Nadal and Karolina Pliskova are the top two seeds in the men's and women's singles draws, respectively.
Osaka is comfortable around the grounds at Flushing Meadows, where she has been spending time, and even occasionally practicing, since she was a kid: She was born in Osaka, Japan, so represents that country, but moved to the US when she was 3, has dual citizenship, and used to live on Long Island.
"I played good today, so I just hope to carry that on to my next match", Osaka said.
Sky Sports Tennis will have all the action from the US Open covered via our website www.skysports.com/tennis with our live blogs and updates throughout the fortnight as the Grand Slam year reaches its climax. Osaka would close the match out with a break of serve in the seventh game, pushing through in just one hour and four minutes.
Kerber, however, barely got that chance as Osaka, who past year had been described by 23-times grand slam champion Serena Williams as "very dangerous", put her on the back foot with her aggressive play.
"It was not my day", Kerber summed up.
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