Novak Djokovic has claimed that Rafael Nadal is the "biggest rival" that he has faced in his tennis career.
The world number one could face the Spaniard in the final of this year's Mubadala World Tennis Championship, with both players starting their campaigns today.
"He's a great champion on and off the court and he's someone who fights for what he achieves," Djokovic told reporters.
"He deserves that, I've played more than 40 times against him on many different occasions and he's probably the biggest rival I have had in my life.
"I enjoy our matches and I enjoy the challenge when I face him on court, especially on clay, because it's the ultimate challenge."
Djokovic is the current World Tennis Championship holder, having beaten David Ferrer in the final in 2013.
