Rafael Nadal starts his pre-retirement farewell tour against Carlos Alcaraz on Thursday at the Six Kings Slam, and Novak Djokovic faces Jannik Sinner at an exhibition event that awards money but no ATP ranking points and is Saudi Arabia's latest foray into tennis.

Rafael Nadal admitted his fitness level was well off the tour's top players after falling Thursday in straight sets to fellow Spaniard Carlos Alcaraz, who described the exhibition match as "a little bit sad".

Rafa Nadal will have one last chance to face old rival Novak Djokovic before the Spaniard calls time on his illustrious career with the pair set to meet at the Six Kings Slam exhibition event in Saudi Arabia.