Novak Djokovic has fought back to dig out a 6-7 (4-7) 6-1 6-4 win over rising Czech star Jakub Mensik and advance to the semi-finals of the Shanghai Masters.

The four-times Shanghai champion managed to raise his level after losing the first set on a tie-break and never looked back as he beat his 19-year-old opponent to enter the semi-finals for a ninth time on Friday.

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

"Jakub is somebody that I've been following the last three, four years...

"We trained at my tennis centre in Belgrade and in Montenegro as well. I could see today why he's one of the best servers we have in the game. He's only 19, so I think the future is very bright for him."

The Serb, who played his first match on the ATP Tour over a year before his opponent was born in September 2005, initially struggled to counter Mensik's serve but found his rhythm and broke for a 5-4 lead in the first set.

But the 24-time grand slam champion allowed his unseeded opponent to hit back instantly, with Mensik going on to clinch the tiebreak as Djokovic hit a forehand into the net.

The 37-year-old did well to recover, though, taking advantage of a string of unforced errors from Mensik to hold serve on either side of an early break as he took a 3-0 lead in the second set.

Mensik, playing in his first ATP Masters 1000 quarter-final, then had no answer to the relentless accuracy of Djokovic's forehand and was broken once again as the fourth seed levelled the match.

Djokovic won the first point of the decider with a pin-point forehand at the end of an epic 33-shot rally and ran Mensik ragged with his power and precision.

He broke for a 3-2 lead, which ultimately proved decisive despite an apparent knee issue for the Serb late in the match.

Djokovic is looking to win a record-extending fifth title in Shanghai, as well as his first ATP Masters 1000 title since his victory in Paris last year.

He will face world No.7 Taylor Fritz in the semi-finals after the American defeated Belgian David Goffin 6-3 6-4 earlier on Friday.

In the other semi-final, world No.1 Jannik Sinner will take on Czech Tomas Machac.