Osaka apologises to children after struggling with Achilles issue at US Open

Naomi Osaka continued to impress with her style and staying power at the US Open on Saturday as she shrugged off a late Achilles problem to beat Belgian Elise Mertens 6-3, 3-6, 7-5 and reach the fourth round.

For her latest eye-catching walk-on look, the four-time major champion blended boardroom and locker-room aesthetics in a grey checked blazer-hoodie and wide-leg pants, putting a tailored twist on traditional pre-match attire.

She looked on course for a business-like win after the first set but was dragged into a battle in the third, where she had to fight through the Achilles niggle to advance.

"I wouldn't say it's a huge injury," Osaka told reporters. "I'd say it's this ongoing problem that I've had since the grass season.

"I don't know why it hasn't gone away. I feel I've done a lot of things in order to make it better, but maybe the time off wasn't enough. I'm not really sure what the reason is, but the medical timeout was to address that.

"You know, she massaged my leg and stuff, so that seemed to help, and gave me Tylenol."

The 28-year-old, a two-time champion at Flushing Meadows, reeled off three games in succession to take a 4-1 lead in the first set under the lights of Louis Armstrong Stadium, clinching it with a battling hold thanks to a string of booming serves.

Naomi Osaka was pushed hard by Elise Mertens (right) but eventually prevailed in a three-setter. (Getty Images: Dustin Satloff)

Osaka pushed the 23rd seed hard at the start of the next set with some heavy hitting from the baseline before pouncing with a break in the third game, only to surrender the lead by dropping the next two games.

"Honestly, I can't believe I won this match," Osaka said on court after sealing victory, thanking the crowd for the energy it provided.

"There was a certain point where I put my heart on the court and kept trying in every point. Thank you so much, but honestly, I'm shocked I won. I'm not going to lie."

After levelling the match to force a decider, Mertens capitalised on several unforced errors by a frustrated Osaka to gain the upper hand, but there would be another momentum shift in the match after a medical timeout for the Japanese player.

"I didn't play amazing and it was getting me down, sorry about my behaviour, for the little kids watching," said Osaka, who threw her racket to the floor at one point.

"This is my favourite tournament. I wanted to play well in front of you. I'm just that glad I have the opportunity to play another match.

"I thought I was going to be OK, but clearly I was shaky."

After recovering a break late on, Osaka fired a huge ace to take a 6-5 lead and revved up her forehand in the following game to complete the win and earn a meeting with Elena Rybakina in the next round.