Rising Spanish tennis star Rafael Jodar said he was still "learning how to behave on court" after swearing at a ballboy at the Shanghai Masters, where he reached the semifinals on Friday to make history.

The 20-year-old recovered from a set down to beat Norway's Casper Ruud 4-6, 6-4, 6-3 and become the youngest player to make the last four in the tournament's history.

It was Jodar's first match since being warned for unsportsmanlike conduct after shouting an expletive at a ballboy in the fourth round.

"I was just frustrated with myself and I couldn't control my emotions at that point of the match," he told reporters.

On Wednesday, Jodar was trailing Botic van de Zandschulp after losing the first set on a tie-break when he shouted in Spanish: "Give me the fucking towel!"

Jodar said Friday he knew it was wrong.

"After the match I thought about it and I shouldn't have done that to the ballkid. It was my mistake," he said after his gutsy quarterfinal win.

He added that he wished to apologise to the ballboy.

"As I'm learning how to do the forehand, how to do the backhand, how to serve, I'm also learning how to behave on court, and there's still a lot of things," said the world number 13.

"I'm not perfect."

Jodar is having a breakout year, after starting the season ranked 168th.

Ruud, ranked 24th, saved three match points before double-faulting to hand Jodar victory.

Jodar will play either 2019 Shanghai champion Daniil Medvedev or American Tommy Paul in the semifinals.