Federer's bid to win a 15th Grand Slam faces a tough examination by Karlovic
Roger Federer's bid to win a record-breaking 15th Grand Slam title faces a stern challenge when he takes on big-serving Ivo Karlovic in the Wimbledon men's quarter-finals on Wednesday.
The 6ft 10in Croat has one of tennis's most potent weapons - his huge serve has sent down an incredible 137 aces in his four matches so far, 41 more than anyone else.
Federer has made serene progress up to now, but knows that Karlovic will prove to be a fearsome foe.
Here BBC expert Greg Rusedski, a man who knows a thing or two about big serving, gives a tactical analysis of the threat Karlovic will pose.
HOW DO YOU COPE WITH THE KARLOVIC SERVE?
Ivo Karlovic is a nightmare for anyone to face. His serve is coming down from a skyscraper's height.
It's going to be a rough match for Roger Federer and it will be his first massive test of these championships.
Set ATP record of 55 aces in his loss to Lleyton Hewitt at the 2009 French Open
Has hit 137 aces at Wimbledon so far
6ft 10in tall, wears size 16 shoes
His fastest official serve is recorded at 153mph
Won the Nottingham tournament on grass in 2007 and 2008
Gone out in the first round at the last four Wimbledons
You simply cannot practise returns to the types of serve he fires down.
When I watched his match against Jo-Wilfried Tsonga he had a 77% success rate on his first serve. It was unstoppable.
You have to know where his serving patterns are and be able to get the serve back.
That is one area that Federer has above all the other players, he knows how to anticipate it and he knows how to block it back.
Most sets with Karlovic will come down to tie-breakers and, when push comes to shove, you have one opportunity to take a set, and Federer will be able to do it.
That is why he has a 8-1 winning record over Karlovic and that is what makes him the greatest player that has ever played the game as he is always able to take those opportunities.
HOW GOOD IS THE REST OF KARLOVIC'S GAME? WHAT ARE HIS WEAKNESSES?
He has improved a lot recently. He has worked very hard physically and I know two years ago he hired a full-time physical trainer.
He now gets down low on his volleys and his backhand is stronger than his forehand at the net.
Most people's tactics to Karlovic is to get the serve back any which way they can and from then target his big weakness at the back of the court - the backhand passing shot on the run.
Everyone aims for that because for him it is a one out of 10 shot and you need to take advantage of his movement.
HOW CAN FEDERER UPSET KARLOVIC'S SERVING SUCCESS?
It does play on your mind when somebody gets on top of your serve.
Karlovic has already served an outstanding 137 aces in four matches
The cleverer players have tactics to seize on any doubts or weaknesses and there is no-one better at it than Federer.
He will probably change his return-of-serve stance, he will stand further back, move around tactically and position himself so that he won't be standing in the same place all the time.
I relied on my serve a lot and it does play on your mind when somebody throws your biggest weapon off course.
Late in my career it did start to frustrate me when players did that, but he has got an extra 6-7 inches on me and rarely loses his serve.
HOW GOOD HAS FEDERER BEEN SO FAR?
Federer is very relaxed, calm and life cannot be anything but perfect for the man right now. He won the French Open, he just got married, he is expecting a child this summer - everything looks great.
If you put the full package together as he did at the French Open it's a frightening prospect for anyone.
The other thing that's fascinating about him is that his back problem is making him play offensive tennis.
HAVE YOU SPIED ANY WEAKNESSES IN FEDERER'S GAME?
His serve and volleying coming forward hasn't been that impressive. That is the one area that everyone talks about him getting better at.
Also, I saw him in earlier rounds not winning too many points on his second serve, but his exceptionally high percentage of points won on his first serve meant it didn't cost him that much.
To really try and hurt those sorts of areas you have to have a game where you can mix it up and bring him forward.
There are very few players who can actually do that to Federer, there is one exception though and that is Andy Murray.
I think Federer will win in four sets.
Greg was talking to BBC Sport's Paul Birch