Over on court 17, Oscar Otte has wrapped up victory over Andreas Seppi in four sets.

The German takes the final set 7-5 after Italian Seppi kept himself alive by taking the third.

And that means Britain's Emma Raducanu will be up shortly...

A bullet first serve down the centre and it's another straightforward service game for Djokovic, who moves within one game of the first set. The world number one is beginning to find his stride now.

And Novak Djokovic is back level on Arthur Ashe.

A real drop in Nishikori's levels costs him the game and his early good work is cancelled out.

The first set is within sight for Nishikori, but he has a battle on his hands to hold again here.

A couple of errors hand Djokovic 0-30.

Djokovic keeps himself in touch in the first set.

Nishikori demonstrates his early confidence with an excellent forehand winner to close to 40-30, but the world number one gets the job done.

A crafty forehand from Djokovic denies Nishikori on a second game point, before the Japanese world number 56 rescues a break point with an ace.

Djokovic strikes into the net and it's another impressive hold under pressure for Nishikori.

Laura Robson
Former British number one on BBC Radio 5 Live Sports Extra

I love watching Nishikori play, his game and his attitude. But he hasn't been able to stay physically healthy enough to compete at the very top level in the last couple of years. It is tough to see. He always seems to be struggling with some sort of injury.

He has had better results this year - he reached the Washington semi-finals after the Olympics - and that's because he's been able to play more. He needs the matches in his legs.

A Nishikori backhand just misses the line and brings Djokovic to deuce. A chance for the world number one to respond...

Great point! Nishikori closes in on the net to apply the pressure but Djokovic races around the court to keep his volleys alive before slotting a passing shot to the left of his opponent for 15-30.

Djokovic is clearly unhappy about his start and mutters to himself before serving.

No problems in this game. A volley completes a hold to love.

Can Nishikori consolidate his early advantage?

A delightful backhand down the line moves him to within a point, before an ace wraps things up. He's made a great start here.

Over on court 17, Andreas Seppi and Oscar Otte remain locked on serve at 2-2 in the fourth set.

Otte won the first two sets but Seppi dominated the third to leave Emma Raducanu waiting a bit longer for her match to begin...

Well, well, well.

Having saved two break points on serve, Kei Nishikori has the early break!

Now then, another double fault from Djokovic makes it 0-30.

An error from the world number one - and suddenly Nishikori has three break points!

A wayward shot costs Djokovic his first opportunity and Nishikori wins an excellent point with a volley following a quick-fire exchange at the net to reach deuce. Some good serving from there seals the hold.

Djokovic moves to 15-30 on Nishikori's serve with a backhand winner, and after a lengthy rally Nishikori nets. Two early break points for the world number one.

It's a comfortable start on Arthur Ashe for Novak Djokovic, who cruises to a hold. A double fault at 40-0 was his only blip.

The next hurdle in Novak Djokovic's quest for a historic 21st Grand Slam men's title is under way.

BBC Radio 5 Live Sports Extra

You'll be able to listen to build up and live commentary of Emma Raducanu's match on BBC Radio 5 Live Extra.

Coverage will begin at around 19:00 BST and will be available at the top of this page and on BBC Sounds.

Laura Robson
Former British number one on BBC Radio 5 Live Sports Extra

Former British number one Laura Robson speaking on BBC Radio 5 Live Sports Extra following Emma Raducanu's second round victory...

Emma Raducanu played a phenomenal match. She was under so much pressure towards the end and handled it well.

I can't believe she is 18 years old. I don't want to be on the hype train but how can you not when she is playing this way?

I haven't seen many bad days from Emma recently and her base level is is right up there with the best players.

The ceiling is so high for her.