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".

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.

The last time Nadal faced 24-time Grand Slam champion Djokovic was in the second round of the Paris Olympics, where he lost 6-1 6-4. Nadal, who also teamed up with Alcaraz in the men's doubles at the Games, could do so again at the Davis Cup Final 8 in Malaga from Nov. 19-24.