Rafael Nadal has said he feels 'sorry' for Roger Federer after the Swiss superstar was knocked out of Wimbledon in the quarter-finals by South Africa's Kevin Anderson.

Anderson, number eight ranked in the world, came from two sets down to beat the eight-time Wimbledon champion yesterday, and will now face American John Isner in the semi-finals on Friday.

"I’m sorry for him, of course," said Nadal.

"But is impossible to win always, even for him (Federer). But that’s how works this sport. Even more here on grass. Just feel sorry for him and congrats to Kevin. Can’t say anything else.

"I was playing tennis. I was not thinking much about what was going on in the other court," said Nadal, who at the same time as Federer, endured a five-hour battle against Argentina's Juan Martin del Potro on centre court.

"I am not that good to have the chance to think about all the things going on outside. I have a tough work having Juan Martin in front."

Rafa took the first set against Del Potro before falling behind by two sets to one, and then clawing it back to win the game 5-7, 7-6, 6-4, 4-6, 4-6.

The second semi-final will see Nadal line up against former world number one and three-time Wimbledon champ Novak Djokovic.

The man from Serbia beat Japan's Kei Nishikori 6-3, 3-6, 6-2, 6-2 to reach the Wimbledon semis, setting up the clash against Nadal on Friday.

Djokovic's fall from grace found him ranked at 21st in the ATP World Rankings before the start of the tournament, but his path back to the top is being cleared, and he will face this year's French Open champion Nadal for the 52nd time in the pairs career.

So as Nadal and Djokovic continue their quest for yet another Wimbledon title, it's time for Federer to head home.