What Roger Federer can do is crazy, says Tommy Haas

and faced each other on Saturday at the Hopman Cup in Perth. The former world No. 2 believes the rivalry between the Swiss and the German is a great one: I always watch them playing.

Rogers game is not just about power and serve, he controls everything. He has fast legs, he uses angles and he is comfortable at the net. When Federer plays well, Zverev struggles to compete with him. Roger is one of the best players in history if not the best.

What he can do is crazy, Haas told Sport.cz Haas also commented on potential: I know he was a world No. 1 junior player. He plays slightly differently than most of the Slovakians.

His backhand is difficult to describe. However, his forehand is amazing. When running, he can play so well. But he struggles mentally. He is very hard with himself on court. I can understand it because tennis is a busy single sport.

If he earns confidence avoiding injuries, he can win the biggest tournaments and beat the best players. He is definitely a dangerous lefty. On , a former Slovakian player, Haas concluded: Karol was on the Tour for several years.

I also know he could not play the Davis Cup final for being alleged of doping and so he had to get a ban. It must have been difficult for him. The country featured a lot of good and talented players. Slovakian tennis school is clearly a quality one.

Slovakians have a good technique ana great ball control. ALSO READ: Rafael Nadal: Worse players than David Ferrer won a Grand Slam title