The 19-year-old, ranked 54th in the world, claimed his first title on the ATP Tour with an outstanding display of powerful tennis.

The final was delayed by almost six hours due to heavy rain and when the players emerged it was clear that Djokovic had an eye infection. He used eye-drops during a changeover in the first set.

Mensik started strongly, breaking Djokovic's first serve game to go 2-0 up and the tall, big-serving Czech was dominating until, at 4-2 Djokovic broke back when Mensik found the net.

The set remained on serve from then on, but in the tie-break Mensik's powerful serve, with two aces, put him in charge from the outset. He opened up a 5-0 lead and although 24-time Grand Slam champion Djokovic fought back the youngster sealed the set with an overhead volley.

It was the first set that Djokovic had lost in the entire tournament. Twice he lost his footing and ended up on his back, and he changed his footwear between sets.

Mensik had beaten Djokovic by the exact same margin in the first set of their only other meeting in Shanghai before losing in three sets.

This time, however, the momentum appeared to be with him.

The second set was a nip and tuck affair, though, with neither player able to break. Once again Mensik's power proved decisive in the tie-break and when Djokovic went long on a return to hand him victory he fell to his back in celebration.

"To be honest I don't know what to say. It feels incredible, obviously," Mensik said in his on-court interview.

"It was probably the biggest day of my life and I did super, which I'm really glad (about), to show the performance and keep the nerves outside of the court before the match.

"I feel just super happy and I think that the feelings will come later," he said.

Mensik has made no secret of the fact that he grew up with Djokovic as his idol and after receiving the trophy he said that he started his career in the hope of emulating the Serb.

"There is no harder task for a tennis player than to beat you in the final of a tournament," he said.

"I am pretty sure that this was just the first of many," Mensik added, before revealing that he had been close to pulling out of the tournament before his first match due to a knee injury before last minute physiotherapy produced the desired results.

- No excuses -

After the match Djokovic admitted he had not felt at his best.

"It's unfortunate for me. Two tiebreaks, just very weird match, weird day with rain delay and all the things that (were) happening. Honestly, yeah, I didn't feel my greatest on the court, but it is what it is. Nothing to take away from his victory," he said.

Asked specifically about his eye problem, the Serb said: "I really prefer not to talk about -- there are quite a few things, but I prefer not to... just congratulate him. That's it. I don't want to sound like I'm giving excuses here for my loss."

Djokovic said he had spotted Mensik's talent a few years ago and invited him to train at his club.

"I saw him play when he was 15 or 16 and invited him, we had some training blocks together. He was training at my club in Belgrade, and, you know, to see his development and evolution is really great, amazing," he said.

"Never really happy to lose, but he's one of the very few players that I would be happier to lose to, to be honest."

The match between the 37-year-old Djokovic and Mensik was the biggest age gap difference in a Masters 1000 final and the biggest age gap of any tour-level final since 1976.