Djokovic proves staying power to set up Shanghai Masters semi-final meeting with Fritz
His victory over the Czech Republic's Jakub Mensik came a day after one of his greatest rivals and contemporaries, Rafael Nadal, announced he would retire in November.
The Serb, who is aiming for a 100th singles title in Shanghai, will next face world number seven Taylor Fritz after the American beat Belgium's David Goffin 6-3, 6-4 earlier on Friday.
"Overall, big battle, it took the best out of me to win it," said Djokovic after the match.
Both Djokovic and 65th-ranked Mensik brought serious firepower to the court, their hard-hitting rallies driving the stadium wild with tension.
The first set was closely fought, with Djokovic's break in the ninth game matched immediately by Mensik in the next.
The veteran pulled ahead early in the tiebreak, but seemed to lose focus, allowing the 19-year-old to come from behind to win.
The 24-time Grand Slam champion dominated the second set though, breaking in the second and sixth game.
Mensik recovered to pile on the pressure again in the third, but Djokovic got a crucial break in the fifth game to be crowned winner.
Djokovic called his opponent "one of the best players we have on the tour".
"He's only 19 and he's already playing at such a high level, managing situations of high pressure so well for someone who doesn't have much experience," he said. "I think the future is very bright for him."
Speed, precision, strength
Fritz quickly took charge in the first set, breaking the 66th-ranked Goffin in the second and sixth games.
Underdog Goffin - who knocked out second seed Alexander Zverev in the last round - looked stronger in the second set, but ultimately couldn't replicate his feat with another top-10 scalp.
It is the first time US Open finalist Fritz has progressed past the third round in Shanghai.
He has lost to Djokovic at every one of their previous nine encounters.
"We've played a lot of times and I've never gotten him... (So I'm) waiting for that time that, you know, maybe I can get a first win on him," said Fritz.
"I'm only getting better and better, and I'm excited for the opportunity to play him."
Djokovic said he expected the match-up to be tight.
"It's the final stages of a tournament, anybody can beat anybody. I'm going to need all of my speed, all of my precision, all of my strength," he said.
World number one Jannik Sinner booked his place on Thursday, as did Mensik's compatriot Tomas Machac, who upset four-time Grand Slam champion Carlos Alcaraz.