Up next on Centre Court is the people's favourite, Roger Federer, against world number 84 Alexandr Dolgopolov.

Federer has never lost to his Ukrainian opponent in three meetings.

Dolgopolov is hoping to reach the second round for the sixth consecutive year. He's reached the third round twice before.

Federer, the world number three, will take the record for the most match-wins at Wimbledon in the Open Era if he is victorious today. It will be his 85th win, and he will move ahead of Jimmy Connors if he makes it.

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Brydan Klein looks set to rue those missed opportunities on the Sugita serve as he falls 5-1 behind in the tie-break.

The Briton battles back though, levelling at 5-5, forcing his opponent further behind the baseline.

But he's run out of steam and Sugita sets up set point and converts on Klein's serve as the Briton slices into the net.

Last year's finalist Milos Raonic is over the first hurdle at Wimbledon.

He's registered a 7-6 (7-5) 6-2 7-6 (7-4) win over Germany's Jan-Lennard Struff.

Just the 20 aces for Raonic during that victory.

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Novak Djokovic speaking on BBC One: "It was great to be back on Centre Court. It's the cradle of the tennis history and it's a special feeling to walk on. It brings back many memories. It's nice to be back but you never like to end a match this way.

"He had issues walking on to court. I tried to focus on my game plan, I was serving well and when it mattered I made a break. But you could see he wasn't moving. I'm sure he didn't want to finish this way. It's Wimbledon and it's a big tournament. I wish him all the best.

"I'll try and rest and recover now after a lot of matches at Eastbourne."

Martin Klizan's early departure means this guy is up next on Centre Court.

Hey, Roger!

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She's struggling in the second set, but wildcard player Katie Boulter, who is making her senior Wimbledon debut today, took the first set in her match against American Christina McHale.

Better from Boulter as she wins her first game of the set but McHale comes right back to re-establish her three-game lead.

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Meanwhile, on court 18, the first break of the match has gone Marcos Baghdatis' way. James Ward saved three break points but Baghdatis kept coming.

The Briton currently trails 3-5. You can watch that game here.

WHAT A POINT!

Sugita and Klein drag each other round the court with a series of flicks and volleys, the Japanese finally getting the better of the Briton with a delicate volley that lands deep.

All that means they're heading to a tie-break to decide this first set.

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Easy as you like for McHale now as she consolidates the break with a simple service game to 15.

This set racing away from Boulter.

Over on court 12, Boulter has undone the good work in winning that first set by losing her first service game in the second.

McHale gets three set points, the Brit saves two with a series of well-placed backhands but the damage is done and the American grabs the initiative in the second set.

Another chance for Klein on the Sugita serve at deuce. Can he win the set here and now?

Nope, the Japanese reels off two points to level at 5-5.

John McEnroe
Three-time Wimbledon singles champion on BBC TV

There's got to be a rule for guys who come out clearly not giving or able to give 100 per cent. It's no good for anyone. I think ultimately the player needs to be given advice and made to understand what he is doing to his own reputation and to the sport. There has to be a combination of people to sit him down and give him a way out so he can respect that. There are guys waiting in there, the lucky loser will be looking at this match and thinking he could have played on Centre Court at Wimbledon.

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Novak Djokovic leaves the court and stops for photos and autographs with his adoring fans.

He then starts rummaging around his bag for something. His keys? His wallet?

No, his sweatbands. One lucky lady in the crowd has requested them and the world number four has obliged.

That's it. There's no way Martin Klizan can continue like this.

He holds his hands up and retires from the match.

Novak Djokovic goes into round two after 43 minutes on court.

Few more openings on the Sugita serve for Klein but the Japanese player defends and we're still on serve.

Sugita will have to serve to stay in this first set.

Nick Mullins
BBC commentator

Klizan has been nursing this calf problem for a couple of months. He's looking up towards Carlos Ramos and hoping the umpire suggests to him it's time to head off.

Djokovic speeds into a 30-0 lead before Klizan finds the energy to land a forehand winner.

But that's him spent for this game. Djokovic takes an easy hold.

This is quite painful to watch. Martin Klizan can't run for any shots coming his way.

Djokovic brings up two break points after a double fault and Klizan overcooks a forehand massively to hand the Serb the game.

The big question is: will Klizan continue like this?