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".

The 38-year-old Nadal, who announced last week he would retire from tennis after the Davis Cup Finals in Malaga next month, lost 6-3, 6-3 to 21-year-old Alcaraz in the semi-finals of the "Six Kings Slam" event in Saudi Arabia.

Afterwards he described Alcaraz, winner of the French Open and Wimbledon this year, as an "animal" and acknowledged he was not competing at the highest level.

"I have not much pressure here. I'm just trying to have fun," he told a post-match press conference.

"I know I'm going to play against players that... are in much better shape than me because they are on the professional tour and winning tournaments."

Alcaraz said he knew the Saudi crowd was against him but wanted to let Nadal, winner of 22 Grand Slams including 14 French Open titles, bask in the support.

"It's a little bit sad... It was not a really comfortable moment for me. Obviously I really wanted to win," said Alcaraz.

"Once you step on the court it's no friends, no idols" but "I think this is his moment right now," he added.

Prior to Thursday's match Nadal had made his last singles appearance at the Olympic Games in Paris where his campaign was cut short by old rival Novak Djokovic in the second round.

He had also teamed-up with Alcaraz in men's doubles at the Games and may reprise that role at the Davis Cup Finals.

Nadal will play Djokovic again on Saturday after the Serbian great lost a much tighter match to Australian Open and US Open champion Jannik Sinner, who prevailed 6-2, 6-7 (0/7), 6-4 and will take on Alcaraz in the final.

"Having Novak in front, it's like a nostalgic thing," Nadal said, looking ahead to their battle for third place in the Saudi exhibition.

"We faced each other plenty of times, so it's going to be good fun to play each other again one more time in this match."

As for the Davis Cup Finals next month, Nadal said he was unclear what his form would be.

"Emotionally I'm sure I will be ready. In terms of physically, in terms of tennis level, I mean (there) remains a month" to prepare, he said, adding that he would not hesitate to speak up if he thought he could not compete well in singles.