Naomi Osaka is unsure whether she will be fully fit for the US Open.

The 2018 champion pulled out of the final of the Western & Southern Open on Sunday after picking up a hamstring injury in her semi-final win over Elise Mertens, handing the title to Victoria Azarenka.

The Japanese player is due to face her compatriot Misaki Doi in her first-round match on Monday and hopes to recover in time.

"I never know with that, but I just hope I'm giving myself the opportunity and the chance to have enough time, because I feel like I would know the answer after I played my US Open match," she said.

Osaka is just one four of the women's top 10 playing at Flushing Meadows after a host of withdrawals due to concerns over the coronavirus.

Ashleigh Barty, Simona Halep, Elina Svitolina, Bianca Andreescu, Belinda Bencic and Kiki Bertens all decided not to travel to the United States.

That should give Serena Williams an increased chance of finally moving to 24 grand slam titles, which would equal Margaret Court's record.

She has been stuck on 23 since winning the 2017 Australian Open and has lost her last four grand slam finals, including two at the US Open.

The 38-year-old still has fire in her belly, though.

"I've been definitely proudly stuck here, party of one. I'm pretty happy about it," she said of her 23 grand slams.

"Obviously I'm never satisfied. That's been the story of my career. So it is what it is. I took a year and a half off for a baby. So I don't know.

"Obviously it's something that I obviously think about, I remember," she said. "It was such a great moment. It's something I'll never forget.

"But it's a little bit hard given the circumstances, what happened.

"Taking it into this week, I'm obviously going to use whatever I can from January, think about it, hopefully build up when I play here."