Former Grand Slam champion Juan Carlos Ferrero says it is absolutely impressive what Roger Federer, Rafael Nadal and Novak Djokovic have been able to achieve and how long they have managed to stay at the top of the game. The three greats have marked an era of tennis and are considered the three best players in the history of tennis.Ferrero, who enjoyed a very successful career as he won a Grand Slam and rose to No. 1 in the rankings, beat Federer three times, Nadal twice, and once finished on the winning side against Djokovic. “I was there trying to find the t …