Defending his title from last year, Alcaraz will be looking to celebrate his 20th birthday on Friday with a win against either Croatian Borna Coric or surprise German qualifier Daniel Altmaier in his sixth Masters 1000 event semifinal.

Alcaraz managed to break the Russian’s serve in the seventh game, closing the first set 6-4.

However, the Spaniard began the second set a little distracted and allowed Khachanov to open up a 4-1 lead before he woke up to fight back and win the match in under two hours.

“Winning this match gives me a lot of confidence,” Alcaraz said.

“Karen had been playing at a high level and took me to the limit, it was decided by small details.

“I will train for Friday, but with a lot of tranquillity and calm, I will not be overwhelmed and I have to be fresh.”

Alcaraz has reached 18 consecutive wins in Spanish clay-court tournaments since losing to Rafael Nadal in Madrid two years ago on his 19th birthday.

His winning percentage overall on clay is already the third best since 2010 (82.3%), behind only Nadal and Novak Djokovic.

In the women’s singles, Russian Veronika Kudermetova beat American Jessica Pegula 6-4, 0-6, 6-4 and reach the Madrid Open semis.