"You challenged me in ways no one else could," 20-time Grand Slam champion Roger Federer wrote as Rafael Nadal prepares to retire after Davis Cup.

Rafael Nadal is about to officially head into retirement after the Davis Cup, and with the possibility that we'll see him play his final match on Tuesday, his friend and rival Roger Federer penned a beautiful message to Rafa on social media.

Roger Federer says Rafael Nadal made him enjoy tennis “even more” during a rivalry that spanned 40 matches over 15 years. Writing on social media Tuesday, hours before the start of the Davis Cup Final 8,