Jannik Sinner has saved three break points in this service game.

They are still going...

Jannik Sinner finds his way back in to the game from 30-0 down but Alexei Popyrin finds a big serve to see out the hold.

It's still only one break to the Italian but he is well in control here.

It's relentless from Jannik Sinner.

He holds and it feels like Alexei Popyrin will need to find something quickly to save this match.

Image source, Getty Images

Russell Fuller
Tennis correspondent on BBC Radio 5 Live and BBC Sounds

This match is looking very good at the moment but it's not the contest that we were hoping for.

Alexei Popyrin goes long with a forehand and gets himself in a spot of bother at 0-30.

He then tries to lob Jannik Sinner but the World number one volleys at the net and brings up three break points.

He takes the opportunity and that feels like a significant moment.

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Jannik Sinner gets himself to 40-0 but some good attack from Alexei Popyrin wins him a couple of points.

But the Italian closes out his service game and we begin this second set on serve.

It's a great response from Alexei Popyrin.

He holds his first service game of the second set to get us back under way.

Image source, Getty Images

Jannik Sinner has some heavy taping around his right foot and this is being adjusted by the physio as the players sit down at the end of the first set.

It's a comfortable hold for Jannik Sinner and a professional performance in this opening set.

After just 43 minutes he establishes a lead and it's looking good so far for the defending champion.

Jannik Sinner misses a cross-court forehand which would have given him break point.

Alexei Popyrin then fires in an ace to recover from 15-30 down, before an outrageous winner helps him hold to leave Sinner serving for this first set.

Image source, Getty Images

Image source, Getty Images

Naomi Osaka speaking on court after beating Hailey Baptiste:

"I was really stressed the entire time, I played her [Baptiste] last in Miami and it was probably the most difficult match of the year. I was trying to focus on not giving her any free points.

"I'm excited to be moving better and I feel really healthy.

"Tomasz Wiktorowskihe [new coach] makes me see tennis in a different way, and I think you can see that in how I've played recently."

On her sartorial choices this US Open:

"I think tennis wants more fashion, when I'm talking to other players they seem excited about it. It's cool people are talking about it."

Jannik Sinner hits a 125mph ace to start his service game.

Alexei Popyrin shows aggression to attack the net and volleys to get himself back in the game.

Sinner then misses a forehand after a weak return from Popyrin but follows that up with a second ace of the match.

The Italian can't win the next point as his opponent forces deuce, but he eventually holds.

Popyrin now serving to stay in the first set.

Image source, Getty Images

Not an ideal start for Alexei Popyrin as he double-faults.

Popyrin then goes long with a forehand to give Sinner the lead against the serve.

The World number one can't take advantage though and full credit to Alexei Popyrin who keeps himself in the first set.

Superb tennis from Naomi Osaka, who pulls Hailey Baptiste all over the court before hitting a sweeping forehand winner.

Baptiste then goes long and Osaka holds. One game from victory.

Naomi Osaka breaks for a second time this set.

Hailey Baptiste is on the ropes, but on break point she stays in a long rally before opening up her arm with a sweet forehand winner to force deuce.

But the American lets herself down and goes long with a forehand.

Given how well Osaka has been serving, that feels like a match point. She needs two more games to win.

A superb exchange at the net between Jannik Sinner and Alexei Popyrin.

Popyrin lands a volley to get himself to 30-15. Sinner then double-faults and Popyrin puts some pressure on the Italian's serve.

Sinner shows his class though and wins back-to-back points to hold and keep himself a break ahead.

Brad Stine
American tennis coach on BBC 5 Sports Extra

Sinner has been finding the backhand of Popyrin and he is going to be finding that more often than Popyrin.

Jannik Sinner misses a forehand winner to let Alexei Popyrin back in to his service game.

But the Italian finds a strong backhand to nudge himself in-front before a powerful forehand forces a miss from Popyrin and brings up two more break points.

The Australian does extremely well to save them both, the second coming after a 17 shot rally.

Sinner forces a third break point, which is again saved by Popyrin, and the Australian eventually gets himself over the line to get on the board in this match.

A simple hold for Naomi Osaka means she leads 3-1.