There's another former US Open champion in action tonight as Bianca Andreescu takes on the in-form Caroline Garcia.

Canada's Andreescu, who beat Serena in the 2019 final, has saved an early break point to keep it on serve at 2-1.

Meanwhile, the winner of this match will face Ludmilla Samsonova, who is on a 13-match unbeaten streak.

The Russian has ousted Serbia's Aleksandra Krunic 6-3 6-3.

Tomljanovic can't convert the first break point, but can the second.

She leaves Williams rooted to the spot with a backhand winner. That's her third game in a row and Tomljanovic will now serve for the set.

This is suddenly going a bit wrong for Serena Williams, finding herself 15-30 down with another double fault to her name.

Tomljanovic sends her to-ing and fro-ing from one corner to the next, and finds herself with two break points.

Russell Fuller
Tennis correspondent at Flushing Meadows

That was impressive from Tomljanovic and, just briefly, she's managed to silence this crowd. They will soon find their voice again...

A few more errors are creeping into Williams' game, but she stays toe-to-toe with Tomljanovic to 30-all.

Williams then plants a backhand between the tramlines and Tomljanovic seals the hold with a serve that her opponent cannot return.

Jonathan Jurejko
BBC Sport at Flushing Meadows

Alja Tomljanovic is here to kill moods like an inappropriate gag at a wake.

Appropriately really as the sounds which greets the Australian's break - which was more to do with Williams' unforced errors - is funereal silence.

Well, well, well. Groans from the Ashe crowd as Williams clips the net cord and the ball drops down on the wrong side of the net with the score tied at 30-30.

That hands Tomljanovic a break point before she's gifted the game as Williams fires over.

Image source, Getty Images

Of course, she needs just the one break point, as Tomljanovic puts too much behind a backhand return.

A standing ovation from the crowd and Williams will serve for the set.

Image source, Getty Images

Laura Robson
Former British number one at Flushing Meadows

That was a perfect point from Serena.

That party atmosphere is building...

Williams comes out flying on Ajla Tomljanovic's serve, and brings down an overhead volley with aplomb to earn herself three break points.

Jonathan Jurejko
BBC Sport at Flushing Meadows

Got to admire the crowd's commitment to creating the celebratory atmosphere.

Behind one of the baseline, four friends have each invested in helium to tell Williams she is the G-O-A-T in gold-lettered inflatable balloons.

They might well be deflated and discarded like gig floor debris later if Tomljanovic poops this party.

Russell Fuller
Tennis correspondent at Flushing Meadows

This is a very competitive contest and quite the tussle.

A fabulous hold for Serena Williams. She's really up for this one.

A rally at 40-15 ends with Williams scooping a winner over the net and out of Tomljanovic's reach, and Ashe is on its feet.

Laura Robson
Former British number one at Flushing Meadows

Tomljanovic's serve can break down when she's under pressure.

Oooosh, a peach of a forehand into the corner from Serena Williams sees her win the opening point of Ajla Tomljanovic's service game, but she then flops a forehand into the net with her positioning all wrong.

From there, the American sends a return just wide but Tomljanovic is eventually able to hold to 30.

A missed opportunity, you sense, for Williams.

Russell Fuller
Tennis correspondent at Flushing Meadows

If Serena is able to win tonight and then get into the second week you would think that, at some point, the matches will catch up with her in a major way.

Jonathan Jurejko
BBC Sport at Flushing Meadows

More celebs have turned up here this week than at a Manhattan fashion party promising free fizz and some column inches in a favourable publication.

The A-Listers have been out in force, with Vogue editor Anna Wintour and golf superstar Tiger Woods in Williams' box previously this week.

Today it is R&B diva Ciara and her fella Russell Wilson, who is one of the country's biggest NFL stars.

Elsewhere I've spotted seminal film director Spike Lee, sat below me dressed in a pinstriped suit and flat cap which looks like the uniform at Victorian public school, and actress Rebel Wilson.

Image source, Getty Images

A bit of a murmur bounces around Ashe as Ajla Tomljanovic returns a winner down the line off Serena Williams' serve. I don't think the Aussie was expecting that, as she raises her racquet in a sort of apology.

Williams soon finds herself 30-15 up, her first ace of the day helping her along the way, before a powerful backhand down the line hands her game point.

Uh oh, a double fault hands Tomljanovic a glimmer of hope, but the world number 46 follows that up with a forehand sent too long.

Ajla Tomljanovic starts to show her range in her service game, with some great striking of the ball.

A love hold of her own, wrapped up as Serena Williams crashes a forehand into the net.

Jonathan Jurejko
BBC Sport at Flushing Meadows

As fans bounce towards Ashe through the South Gate they walk through a corridor known as the Court of Champions.

One side is reserved for the best of the best, celebrating the legacy of the greatest winners in US Open history.

Serena Williams has not yet been inducted, but she will surely be once after her retirement.

For now, she is immortalised on the opposite side of the tribute where her name is etched into bronze on the honours board.

Not once. Six times.

Image source, BBC Sport