Naomi Osaka shakes off injury fears to battle past Misaki Doi at US Open

Japan's Naomi Osaka showed no signs of discomfort from the injury that forced her to miss a final on Saturday but she had to battle to beat compatriot Misaki Doi 6-2 5-7 6-2 to reach the second round of the US Open.

A left hamstring injury had forced Osaka to withdraw from the Western & Southern Open final against Victoria Azarenka but the fourth seed's movement did not seem to be affected against Doi on Monday.

The past week saw Osaka, 22, emerge as tennis' torchbearer in protests against racial injustice and she walked out to the court wearing a mask featuring the name of Breonna Taylor, a Black woman killed by police officers who burst into her apartment in March.

Osaka, the 2018 champion, started strongly in the opening set as she broke Doi's serve twice without facing a single breakpoint.

But she struggled with her serve in the second and her unforced errors mounted as the 81st-ranked Doi, who lost to Osaka in their only previous meeting in 2016, levelled the match with a second break.

Normal service was resumed in the decider, however, as Osaka broke her Fed Cup team mate early before sealing the win with a second break.

"It was very difficult and I kind of expected it because first-round nerves and also she's a tough opponent so I knew there was a chance it would get really long," Osaka said in a courtside interview.

"I felt like it could have been better (with my serve) but it did what it needed to do on the very important points so I can't be that mad. I definitely need to practise some more."

Next up for Osaka, who was born in Japan to a Haitian father and Japanese mother and moved to the United States when she was three, will be Italian Camila Giorgi, who earlier beat Alison van Uytvanck 2-6 6-1 7-5.

"She's very unpredictable for me so I guess I'm going to have to be on my toes," Osaka said.