The Italian Open final in May 2006 emphasized Nadal and Federer's status as the sport's greatest stars, according to the ATP. It was won by Nadal, who extended his head-to-head record to four wins and one loss against Federer, after another ill-tempered showing from the Swiss.
Federer, with his seven Grand Slam titles to that point, was up against the upstart. One British coach called him "the toughest b------ the game had ever seen," according to The Guardian.
In Rome, Federer felt the full effect of losing out to his natural-born rival. Toward the end of the final, Federer lost his focus completely, turned to Nadal's uncle Toni and accused him of coaching illegally.
"He was coaching a little bit too much again today," Federer said after his loss. "Yeah, I caught him in the act."
Nadal, still a teenager at this time, offered the older Federer some sage advice. "He has to learn to be a gentleman even when he loses."
Source: ATP, The Guardian, and Tennis.com.