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Osaka overcame Elise Mertens in straight sets on Saturday (AEST) to reach the final at Flushing Meadows but sustained an injury to her left hamstring in doing so.

It gave Azarenka a first WTA singles title since Miami in 2016, albeit in circumstances she would not have wanted.

"I'm sorry to have to withdraw today with an injury," Osaka said. "I pulled my left hamstring yesterday in the second set tiebreak and it has not recovered overnight as I had hoped.

"This has been an emotional week and I want to thank everyone for the outpouring of support."

Osaka's decision not to go ahead with the final capped an eventful tournament for the former world No.1, who initially pulled out of her semi-final to join in the nationwide protests over the shooting of Jacob Blake by police in Wisconsin.

Born to a Haitian father and a Japanese mother, the 22-year-old said she was withdrawing from the tournament to "get a conversation started in a majority white sport".

However, Osaka reversed her decision, with both semi-finals pushed back 24 hours from Friday (AEST), and she emerged on the Grandstand court at Flushing Meadows wearing a Black Lives Matter t-shirt.

"It's been a little bit stressful, I couldn't really sleep last night so I'm glad I was able to play at a good level," she said after beating Mertens.

"I'm really glad I didn't mentally collapse. I was down a break in the second, I was down in the tie-break. I'm really glad I didn't just dip."

The No.4 seed is due to begin her US Open bid against fellow Japanese player Misaki Doi in New York on Tuesday (AEST).