Naomi Osaka revealed she “felt really restrained” during her shock first-round loss to Danielle Collins at the US Open.

The 24-year-old was defeated 7-6, 6-3 by Collins, the current world number 19.

Given Osaka sits 44th in the world rankings, the result may not seem too much of a surprise.

But the Japanese star is a two-time US Open champion, and it was expected she would progress further into this year’s tournament than the opening round.

After her defeat, Osaka told reporters that she was struggling to “hit the shots that I know I can”.

"When I think about having fun, it's like I don't feel restrained,” she said. “Lately I felt really restrained. I'm not really sure why.

“I'm beginning to feel like I can't hit the shots that I know I can, like especially on my forehand… I just don't have the same feeling."

Naomi Osaka and Danielle Collins

Osaka missed a large part of the 2021 season in order to protect her mental health, and has struggled to find her form again this year.

She was knocked out in the third round of the Australian Open, the first round at Roland-Garros and chose to skip Wimbledon. Her latest comments suggest she is some way from firing on all cylinders again.

In contrast, Collins feels she is as fit as she’s ever been, despite not playing since the Ladies Open Lausanne in July.

The American took a break after the tournament to let a neck injury heal properly, and also have an established training block ahead of the US Open.

Danielle Collins at the US Open

"When I made finals in Australian Open, I didn't play any warmup tournaments," Collins said. "That gave me a lot of confidence. I was thinking my best results in Slams have been when sometimes I don't play a warmup tournament.

“I made the quarterfinals of the French back in 2020. Didn't play any clay court tournaments that year before. Just went out and felt really good. I guess it works for me sometimes."

Collins’s next opponent will be Spain’s Cristina Bucșa, a qualifier who earned an opening round win against Kaja Juvan.

There have been a number of other shock exits at the US Open so far, including Simona Halep, Elena Rybakina, Daria Kasatkina and defending champion Emma Raducanu.