"I'm super grateful for the time we spent together and the things I learned on and off the court, but I feel like now is an appropriate time for change," the Japanese 21-year-old wrote on Twitter.
"Appreciate you, forever warmed by you [...] Thank you for everything, it was a blast."
The loss was another disappointing moment in a difficult season for the power hitter since her second major title, with early exits at the French Open and Wimbledon.
The Australian Open win made the Japanese star Asia's first world No. 1 and the first player of any nation to claim their first two Slam titles back-to-back since Jennifer Capriati in 2001.
But after a first round exit from the Dubai Tennis Championships in Februrary, Osaka tearfully said the top ranking and accompanying attention were a heavy burden on her.
Australian Ashleigh Barty later took the No. 1 spot in the lead-up to Wimbledon, where Osaka said after her first-round exit the fun had gone out of her game.
As Barty faltered during the North American hardcourt season, Osaka reclaimed the top ranking in the lead-up to the US Open despite tepid form in Cincinnati and Toronto.