The first real sign of animation from Novak Djokovic as he nets a second-serve return and then shouts something - presumably towards his coaching team.

Zachary Svajda outlasts him in a lengthy rally and then Djokovic goes long - two set points for Svajda.

The first one is saved, but the second will be on Svajda's serve.

Leon Smith
GB Davis Cup captain on BBC Radio 5 Sports Extra and BBC Sounds

I'm not quite sure if he's trying to do breathing exercises or if he's feeling some physical distress. It looks more like physical distress to me. He did say he was struggling with his breathing a little bit in the first-round match, and we're not talking about high levels of heat or humidity here.

He said he was healthy, as far as he knows, but maybe he's aware he's got some sort of bug in his system, I don't know but he's always been like this. He is not someone who will try to hide any discomfort.

He doesn't look like an intimidating presence and I'm sure that will give Zachary Svajda some encouragement.

That mini-break is given straight back as Zachary Svajda nets a backhand.

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The first player to seven points will win the tie-break and Novak Djokovic gifts Zachary Svajda the first mini-break with a backhand that goes long.

Novak Djokovic starts his service game with a double fault, his second of the match. Zachary Svajda almost wins the second point too but is just off target with a cross-court shot at the net.

Straightforward from there for the 24-time Grand Slam champion, who forces a first-set tie-break.

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Andrea Petkovic
Former world number nine on BBC Radio 5 Sports Extra and BBC Sounds

He played the night session almost every other tournament this year, and I wonder if he's now going with the earlier slot to regulate his sleeping patterns more.

He has talked about it in Wimbledon. He will be feeling well, he's playing well, but once he gets to the semi-finals, he feels like his tank is half empty, and if he's playing Sinner or Alcaraz or Zverev, who seem to be in much better shape, maybe that's something that he's trying to adjust, sleeping better, to fill the tank a bit more.

It is very strange to see him play at this time, but it's not a super hot day, no more than about 23 degrees at the moment. And I think if he can win and get through by early afternoon, then he'll probably be all the better for it.

Zachary Svajda will have at least a tie-break in the first set.

Novak Djokovic is looking a little subdued at the moment.

Leon Smith
GB Davis Cup captain on BBC Radio 5 Sports Extra and BBC Sounds

A brilliant performance from Emma Raducanu, she will be absolutely delighted with that result. She has played two matches and only played about two hours of tennis so far, so you would think she will be feeling very fresh.

As for Janice Tjen, she has a very bright future ahead of her and she will have no issues breaking inside the top 100. There are a few things in her game that she can work on, she could do with hitting with a bit more spin and shape as well as controlling the ball a bit better.

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Novak Djokovic finds himself two points away from losing the first set to Zachary Svajda, but eventually holds serve.

Djokovic is down at 45% for first serves in so far - well below his usual level.

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That is some response from Jiri Lehecka.

A second-set bagel gets the Czech on level terms over on court five.

Don't forget, Cameron Norrie will be next up on this court against Francisco Comesana.

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British number one Emma Raducanu, speaking on Sky Sports: "I'm very pleased with how I played today. I had to wait two days to play this second round match so that kind of breaks the rhythm a little bit. I knew I was playing Janice [Tjen], who is a super dangerous opponent and she was playing extremely well coming into this. I knew she would easily put away any ball I put mid-court that wasn't necessarily good enough, so I'm really pleased with how I performed today.

"Janice [Tjen] is playing some really dangerous tennis at the moment. She has gotten through against some really tough opponents and she beat one of the top seeds in the first round, so I'm sure she will build and take a lot of confidence from this week. It was great to play against her and I look forward to the next time."

"It was really good to play in front of you guys on Louis Armstrong Stadium. I loved it once again so thank you so much."

On getting her 26th win of the season, the most victories she has recorded in a single year: "First of all, I think the consistency of doing good practice days every day has helped to build on that. I started to do that after the Miami Open and I was putting in some really good work, but I've also stepped up what I'm doing on and off the court in recent weeks.

"I'm really happy that I have those people in my corner over there and I'm really grateful for them. I'm just looking forward to building day-by-day and trying my best, that's all I can do. I'm putting a lot of trust in the work I'm doing behind the scenes so that's taking some of the pressure off."

Novak Djokovic has 15-30 on Zachary Svajda's serve but Svajda gets the better of a long rally with a rocket of a forehand winner down the line.

A backhand volley winner at the net wraps up a service hold to make it 5-4 to the American.

That was Raducanu's quickest ever Grand Slam win - exactly one hour.

She took 31 minutes to take set one and 29 minutes for set two.

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I'm not sure Novak Djokovic will have had too many morning starts at Grand Slams in recent years.

It's worth remembering, also, that this is the first event he's played since losing at Wimbledon last month.

Still no breaks of serve in the first set.

Victory for Raducanu also makes her the first Briton through to round three in New York.

Can Cameron Norrie follow suit later today?

Jack Draper and Jacob Fearnley also remain in the men's singles draw.

Zachary Svajda has demonstrated in the first 30 minutes of this match that he will be a tough nut for Novak Djokovic to crack today.

Six winners and just the three unforced errors by the American so far, and he holds serve to love for a 4-3 lead.

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Raducanu says she has "stepped up" with what she's doing "on and off the court" in recent weeks.

Well, it's paying off. Full quotes to come.

While Emma Raducanu was getting things wrapped up on Louis Armstrong Stadium, Novak Djokovic was in a touch of trouble facing two break points on his serve on Arthur Ashe Stadium.

But he finds a way to get himself out of danger and levels the first set at three games all.