Cameron Norrie maintains his break lead in this second set.

Sebastian Korda may have earned a break point opportunity there but he looks to be struggling physically, even after that timeout.

The American was squeezing his abdominal area after a couple of those points in that game.

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Victoria Mboko holds to keep herself in contention, but once again the Canadian can't break Barbora Krejcikova.

Mboko has really struggled to deal with the Czech's swerving body serves on second serve, which are often still reaching 100mph. Krejcikova has only lost three points on serve this set.

Daniel Kiernan
Former British tennis player on BBC Radio 5 Sports Extra 2

Barbora Krejcikova is very unassuming, considering what she has achieved. She is such a beautiful ball-striker.

I did think she was the favourite going into this match, despite the seeding telling a different story. She beat Elina Svitolina in Cincinnati, she won three matches, she is just starting to get herself going again.

That's very dangerous for the rest of the field.

Ooft, that was tense.

The pair exchange the longest rally of the match but Cameron Norrie opens the court up with a cross-court forehand, before sending a winner down the line.

That medical timeout has worked wonders...

Daniel Kiernan
Former British tennis player on BBC Radio 5 Sports Extra 2

Sebastian Korda was pointing to his back a few games ago, so I think this is maybe a lower back injury or possibly an abdominal muscle injury.

Here's a bit of turn-up as Renata Zarazua breaks Madison Keys to take a 4-2 lead. Once again Keys breaks straight back though, and we're back on serve.

The sixth seed needs to reign in those unforced errors - she's made 17 so far in the opening set.

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That sums up Victoria Mboko's afternoon. She looks set to hold to love, but then double faults for the seventh time today and goes to deuce.

She digs deep to defend some big forehands from Barbora Krejcikova and manages to hold.

Krejcikova then holds herself despite more great defensive play from Mboko, who hasn't managed to break today.

After making an early breakthrough, Madison Keys then promptly drops serve, and the Australian Open champion is showing signs of frustration.

Renata Zarazua has never beaten a top-10 player in six attempts but she's hanging in well here on Arthur Ashe Stadium, saving a couple of break points in that game to keep things on serve.

That's six break points saved so far by the Mexican.

Sebastian Korda gets himself on the board in this second set, but it was a stodgy hold.

The American really struggled with Cameron Norrie's returning but he won't care how he got the hold, just that he did to stop a five-game losing run. Korda's trainer is now on court, working on the American's hip it looks like.

Daniel Kiernan
Former British tennis player on BBC Radio 5 Sports Extra 2

Petra Kvitova is a competitor and a fighter. She's had an incredible career so she would've wanted to go out in some sort of style.

I'm sure she was realistic with this Grand Slam, in terms of never thinking she would come here and win this event, but she wouldn't have wanted to limp out.

When all said and done, nobody is going to remember this result and people will look back at her career in such an incredible way.

Image source, Getty Images

Image source, Getty Images

Emotional scenes as Petra Kvitova waves goodbye to her illustrious tennis career.

A video is played on the big screen and the 35-year-old is presented with a special poster before speaking to the crowd on Grandstand.

"I wanted to put out a better performance but it was tough to know that maybe it was my last one - [I'm] emotional," she said.

"It's been a long and amazing journey."

She goes on to thank her coaches - past and present - and her family.

Two Wimbledon titles, 29 further Tour-level titles and a genuinely lovely person to boot.

Sebastian Korda must be wondering what's happened.

The American's completely lost his way here.

Cameron Norrie holds to 15, winning his fourth game in a row.

Cameron Norrie, that is brilliant.

The Briton gives himself two break points and he takes the first by forcing Sebastian Korda into a poor return from a sensational passing shot, which the 30-year-old makes no mistake in crashing down the line.

That's four games in a row for Norrie now.

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The mistakes are back for Victoria Mboko - two double faults and Barbora Krejcikova breaks in game two.

The 18-year-old's team are talking her through these difficult moments, telling her to breathe and stay calm, but she doesn't look right.

Krejcikova then holds to build a 3-0 lead.

Some turnaround that.

Cameron Norrie was under the cosh for most of the first set but he wins the final three games to take it.

Sebastian Korda was serving so brilliantly but one poor service game cost the American.

Over on Arthur Ashe Stadium, Australian Open champion Madison Keys is beginning her Flushing Meadows campaign against Renata Zarazua of Mexico. The sixth seed, a former runner-up here, is straight into her stride and breaks the world number 82 in the third game of the match.

Image source, Getty Images

Daniel Kiernan
Former British tennis player on BBC Radio 5 Sports Extra 2

I'm tearing my hair out if I'm Sebastian Korda's coach right now.

He has so many of the tools to become one of the best players in the world, but he is showing a little bit of ill-discipline in shot-making and decision-making here.

He has made three really bad errors in a short space of time to give Cameron Norrie the upper hand.

What a strange start to the second set.

Victoria Mboko misses a sitter at the net with Barbora Krejcikova stranded, before Krejcikova's vibration dampener flies off her racquet as she misses.

Both players, the umpire and staff spend a couple of minutes looking for it, with members of the crowd shouting out where they think it landed. Eventually it's recovered and Krejcikova holds.

Image source, Getty Images