Novak Djokovic said Jakub Mensik was one to watch after being forced to fight from a set down to beat the teenager in a dramatic Shanghai Masters quarter-final on Friday.

Djokovic prevailed 6-7 (4-7) 6-1 6-4 in two hours and 19 minutes to maintain his quest for a 100th ATP Tour-level title, advancing to a record 77th semi-final at Masters 1000 events.

Though he lost control in the second set, Mensik advanced his reputation as an elite server by registering 17 aces to Djokovic's seven throughout the course of the contest, with seven of those arriving in the opening set.

The Czech, ranked 65th, had defeated world number six Andrey Rublev and number nine Grigor Dimitrov en route to the last eight, and Djokovic was impressed by the fight he put up.

"We went toe-to-toe until the last moment," Djokovic said. "I was fortunate to find great serves in the last game, a couple of aces, that helps. 

"I didn't serve that well at the end of the first set when I was serving for it. He managed to turn things around by playing a good tie-break."

Asked about Mensik's potential, Djokovic said: "I could see today why he is one of the best servers we have in the game. 

"He is only 19, the future is very bright for him. He knows that I am always there for him, whatever he needs.

"Jakub is somebody that I have been following for the last three or four years, ever since he played the junior finals of the Australian Open. 

"We like playing each other, we raise the level when we face each other. We've played a lot of practice sets and it's always fun."

Djokovic will now face Taylor Fritz in the semi-finals on Saturday, with a spot in Sunday's final – against Jannik Sinner or Tomas Machac – on the line.