World number Rafael Nadal takes on Juan Martin del Potro in the fourth round of Wimbledon in what could be a thriller at the Centre Court.  

The defending champion Nadal hasn't dropped a set in the three matches he has played at Wimbledon so far. However, all that could change against Del Potro.

The Argentine was once a world number four, back in 2009, but injury problems meant he crashed. However, he has been working his way back up the rankings ladder, and could pose Nadal quite some trouble on the day.

And Nadal knows it. The Spaniard said, I think he's a complete player. It is always is a big challenge to play against him. He's a very tough opponent and one of the best players of the world.

His ranking is much better than what the ranking says today. His level is much better than what the ranking says today. If we talk about level, when he's healthy, he's a top-five player.

However, Del Potro is under no illusions and is aware of his challenge.

It's not easy to be top five but I'm improving my game every day. That's a good sign for the future. But, of course, I want to be at the top as soon as possible. But it's a long, long road.