Aryna Sabalenka finds a way through in the end to back up the break and stay in front.

Camila Osorio can't make the most of the first break point, but she stops Sabalenka from wrapping up the hold by finding the line with another brilliant winner.

The double faults are flying. This time it's Aryna Sabalenka's turn and she's not best pleased with herself.

That forehand winner down the line from Camila Osorio won't do much to improve the Belarusian's mood.

Harriet Dart saves two set points with consecutive forehand winners - the second a thunderbolt of a short cross-court return - before Peyton Stearns nets a routine forehand and smacks her racquet in frustration.

At the fourth time of asking, the American clinches the opening set 6-3.

Aryna Sabalenka won't get an easier break of serve. Five double faults in two service games for Camila Osorio and that one proves costly.

Image source, Getty Images

A yell of despair from Aryna Sabalenka. That's a poor miss from the world number one and we're back to deuce.

But not for long. Yet another double fault from Camila Osorio.

Camila Osorio goes a little too big on the second serve, double faulting to hand Aryna Sabalenka the first break point of the match.

Trailing 40-15, Peyton Stearns buries a forehand winner before an untimely double-fault from Harriet Dart ushers in duece.

The Briton sends a simple forehand long at advantage and although she saves a break point to force a third deuce, she can't hold off the American. Down-the-line forehand for advantage before Dart's backhand abandons her.

Stearns will serve for the opening set.

Camila Osorio has less luck on serve. Aryna Sabalenka is returning well and she stays alive in the game thanks to a timely backhand winner down the line. Deuce.

A comfortable opening service game for Aryna Sabalenka. Two aces, one unreturnable serve and then a serve-volley combo to finish it off.

Image source, Getty Images

Since winning the 'Sunshine Double' at Indian Wells and Miami in spring, Aryna Sabalenka has a 17-7 win-loss record.

That includes a chastening French Open quarter-final exit and a disheartening defeat in the Wimbledon fourth round.

And she hasn't had the best of build-ups to this year's US Open with early losses in both Cincinnati and Toronto.

However, Sabalenka has not lost in the first round of a Grand Slam in almost seven years and she has reached the fourth round in her last 17 major appearances.

What happened there?! After pushing the ball into the net again, Aryna Sabalenka completely mis-hits Camila Osorio's next serve and sends the ball flying skywards on Arthur Ashe Stadium.

Some very early signs of frustration from Aryna Sabalenka as she pelts the ball into the net.

But she pulls us back to deuce with some massive forehands that Camila Osorio can't cope with.

Nice from Camila Osorio as she sinks a lovely cross-court backhand winner, recovering from an earlier double fault.

Ah. But there's a second double fault to drag Sabalenka to 40-30.

And then a huge return from the top seed on Osorio's second serve. Deuce.

Two more break points saved by Harriet Dart, who was down 15-40 before coming through a second deuce with an ace and a wide serve returned into the net to hold.

Peyton Stearns holds to 30 to keep the first set on serve.

Image source, Getty Images

Aryna Sabalenka is just about to get her title defence bid under way.

Things haven't gone to plan for the top seed lately and her position as world number one is under serious threat.

With 2,000 points to defend at Flushing Meadows and Kazakhstan's Elena Rybakina now trailing by only 434, Sabalenka must win again to extend her 99-week run as the world number one.

A first-round defeat today would be far from ideal for the two-time defending champion.

Camila Osorio will serve first to get us up and running...

*denotes next server

Marion Bartoli
Former Wimbledon champion on Sky Sports

I was so sad. It was so sad to watch. To see Novak Djokovic suffering that much was breaking my heart.

The way he was physically and health-wise struggling is what worries me the most. Health should come first and surely it should come first for Novak.

You can see how much he is torn with the decision whether he should keep on playing, whether he should retire.

Image source, Getty Images

The defeat ended Novak Djokovic's latest bid for a standalone record 25th Grand Slam singles title, which would move him clear of Australia's Margaret Court.

Afterwards Djokovic revealed he had been hampered by a "health condition" which has been "bothering" him in oppressive conditions over a number of years and, discussing the issue with reporters shortly after losing to Navone, called it a "serious issue".

"It's something that I have been carrying pretty much every match this year - vomiting, throwing up," Djokovic said.

"And then my whole body starts to collapse basically, cramping and pain - the back just gave up in the end and shoulder, and I couldn't close it out."

The first set on court 12 is going with serve.

Harriet Dart had four opportunities to break in the opening game against Peyton Stearns but couldn't capitalise.

The Briton has since saved a break point in her second service game before Stearns eased through a hold to love.

Before we dive into any of today's action, let's recap on what happened on Arthur Ashe Stadium overnight in the UK.

Novak Djokovic, the 24-time Grand Slam champion, broke down in tears during a shock first-round defeat by Argentina's Mariano Navone.

The 39-year-old was overcome by emotion in the deciding set of a dramatic match where he struggled physically throughout and had to vomit into a towel bin.

It was the first time the Serb had been beaten in the opening round at Flushing Meadows and the first time he had lost in the opening round of a Grand Slam tournament since 2006.