"The main weakness is Novak"

Ancic says "it must be a nightmare" to play Djokovic. Despite Ancic's retirement the two remain good friends even practicing together ahead of the US Open Final in 2015 which Djokovic won in four sets against Roger Federer.

Photo credit: Mario Ancic (L) of Croatia and Novak Djokovic (R) of Serbia and Montenegro embrace after Mario Ancic won their match during day seven of the Wimbledon Championships 2006 - Photo by Phil Cole/Getty Images