Carlos Alcaraz admitted Wednesday it will be a "difficult moment" facing "idol" Rafael Nadal when the Spanish superstars clash for the final time.

Nadal, the winner of 22 Grand Slam titles, announced last week that he will retire from tennis after the Davis Cup Finals in Malaga next month.

On Thursday, however, the 38-year-old will take on Alcaraz for a place in the final of the "Six Kings Slam" exhibition event in Riyadh.

"Since I started to play tennis, I'm watching his matches. I look up to him and I grew up watching him. He was my idol, he is still my idol," Alcaraz said on court after defeating Holger Rune 6-4, 6-2 to set-up an emotional meeting with Nadal.

"A really important person for my life, for my career and he was one of the reasons that I wanted to become a professional tennis player. Knowing he is going to retire is pretty tough for everyone, for me."

Alcaraz, the reigning French Open and Wimbledon champion, added: "I will try to enjoy sharing the court with him once again, but I think it is really difficult for everyone."

Nadal and Alcaraz have met three times on the main ATP Tour with the soon-to-be retired veteran holding a 2-1 advantage.

The Spanish pair played doubles together at the Paris Olympics this year and they are expected to team-up again at the Davis Cup Finals.

Nadal also made his last singles appearance at the Games on the same Roland Garros courts where he won 14 French Open titles.

His Olympic singles campaign was ended by old rival Novak Djokovic in the second round.

Thursday's other semi-final in Riyadh sees Djokovic tackle Australian Open and US Open champion Jannik Sinner.

World number one Sinner eased past Daniil Medvedev 6-0, 6-3 on Wednesday.

"The semi-final against Novak will be a different match than the ones we played in the past," said Sinner who defeated the 24-time Grand Slam title winner in the Shanghai Masters final last Sunday.

"We know each other quite well and we know what to expect from each other.

"I don't know what's going to happen tomorrow, but I am just trying to play the best possible tennis, and we will see how it goes."