And Federer only needs one. Unfortunately for Anderson it is his own undoing, tension getting the better of the South African as he double faults.

Maybe Federer had got into his head by moving forward on his second serves.

Image source, Getty Images

Now it is Anderson whose serve starts to creak like the old wooden door leading to your nana's pantry.

The South African is already 0-30 down when he challenges a call which judged a cross-court backhand to be wide.

Duh-duh-duh-duh... the heartbeat over the PA system adds to the drama... it is wide!

Three break points for Federer...

Federer is soon level at 30-30 - reacting with a grunt - and then moves within a point of the game when a stretching Anderson goes long from the baseline.

A change of pace from Federer as he draws in Anderson with a drop shot, then sweeps another backhand past his prey and down the line.

Game.

Chance? Anderson wins his first point on the Federer serve for 0-15, then forces the Swiss into batting a forehand into the net for 0-30. Cue groans from many in the O2 crowd...

The pair become embroiled in a baseline scrap, both men hitting the white paint with incisive forehands.

Anderson challenges one.. he's wrong... and then bats a backhand into the net when the point is replayed...

Anything Federer can do... Kevin Anderson whistles through a hold to love. Simple. The kick serve was causing problems for Federer, allowing Anderson to move in with some hefty forehands.

Image source, Reuters

Roger Federer's serve is more accurate than those expensive Swiss watches he advertises.

One down the middle gets the game off to a flyer, Anderson smacking a shot from the baseline long for 30-0.

Another ace down the T brings up three game points, then a trademark one-handed backhand (the sort that get Fed Heads drooling) clinches another easy hold.

Not dropped a point on his serve yet...

Any momentum which Federer thinks he's got quickly disappears in deuce. Anderson keeps his composure and holds with another whopping ace.

* denotes next server

Anderson's serve is vastly important - although there's much more to his game than just a cannon of a right arm, of course - and he starts his second service game with another ace.

Federer's movement looks good though, scurrying into position to hit a forehand winner for 15-15, before Anderson wrestles back control at 40-15.

A pair of sliced returns from Federer cause him problems and force two errors. Deuce...

Leon Smith
GB Davis Cup captain on BBC Radio

It's a tough one to call because Anderson is in great form and Federer desperately wants to win to top the group.

Image source, Getty Images

Can you match that Rog? Federer serenely moves 30-0 ahead.. nope, make that 40-0... and then finishes off an impressive hold when he springs forward and volleys in an overhead.

Cold? Anderson is warmer than a freshly-cooked bagel. Which, incidentally, he fed Kei Nishikori on Tuesday.

Big Kev starts with a thumping ace... then lands another missile of a first serve which Federer can't get back to safety.

A hefty second serve goes the same way from Federer's racquet and, after the Swiss fires off a warning sign with a forehand winner, Anderson pins him down again to hold.

Leon Smith
GB Davis Cup captain on BBC Radio

Kevin Anderson has changed his persona on court. He's very nice off the court but he has started pumping his fist a little bit more and now engages with his team in a bid to find that extra edge.

It's not his real character but it's working for him and he now believes he can win a Grand Slam.

Game on! Anderson to serve. Federer hoping to catch him cold...

Federer won the toss and, like he has regularly recently, decided to receive first.

Who's the favourite here then? Many think it is Anderson. He's had the season of his life and has played superbly here this week.

Which Roger Federer is going to turn up tonight then? The ageing imposter who got wupped by Kei Nishikori on Sunday? Or the ageless icon who eased past Dominic Thiem on Tuesday?

BBC Radio 5 live

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The sound of a thumping heartbeat is played out over the PA system as Anderson, looking relaxed and smiling, walks out on court first.

Federer, a little more apprehensive it must be said, follows him a few moments later. He does manage a reassuring grin when he takes the hand of the young mascot though.

Image source, Getty Images

Andrew Castle
BBC Sport tennis commentator

It's been a while since Federer has won here at the O2 and he'd love to make it number 100 here.

Roger Federer tells Sky Sports: "I feel better today after winning my second match. That showed there's nothing wrong with my game, it's just an energy level question and that's what I am going to bring tonight.

"I will focus on my serve, see what I can do on the return and bring a positive mind-set."

The O2 turns into a nightclub in these moments before the players come out. Cornea-damaging EDM music, flashing LED lights, perhaps some couples smooching in a dark corner...