Novak Djokovic wasn't happy when the umpire slapped him with a warning at the Shanghai Masters.

Facing Alex Michelsen in his opening match, the world No. 4 ran down the shot clock early in the second set and received a time violation.

He immediately questioned chair umpire Mohamed Lahyani over the call, though he was reminded that the clock was now automatic.

Djokovic is back in action for the first time since he helped Serbia beat Greece in the Davis Cup three weeks ago. The 37-year-old had a tough test as he faced Michelsen on Saturday, going down an early break.

He stormed back to win the first set in a tiebreak but still didn't appear too pleased with his level while serving at 1-1 in the second. After sending the ball long, the 24-time Major champion shouted to himself in frustration.

And things went from bad to worse as he was given a time violation warning while serving at 15-30 down. Djokovic bounced the ball away and turned to the umpire.

"Why do you do this man? Why do you do this man?" he asked.

Lahyani reminded him: "It's not in my hands." The Shanghai Masters is using the new automated shot clock, which begins three seconds after the previous point ends. Players then have 25 seconds to serve.

It means the umpire can't use discretion to delay the start of the countdown for a long rally or for crowd noise. And commentator Nick Lester thought Lahyani felt bad for warning Djokovic.

"It was almost like he didn’t want to do it. He said it with a whisper didn’t he, Mohamed Lahyani there. He knew what was coming," he remarked.

Co-commentator Arvind Palmer replied: "It’s not ideal, is it? But it’s enforcing the rules.

"That’s half the problem, the rules aren’t always enforced. Starts automatically, goes over 25 [seconds], what are you supposed to do?"

Djokovic didn't appear too bothered by the warning. He fired down an ace to get to 30-30 and promptly held serve. The world No. 4 isn't the only one who has struggled with the new automated shot clock.

Carlos Alcaraz took issue when it was trialled at the Queen's Club in June, calling it "crazy" and "something bad". He also threatened to speak with the ATP.

The Spaniard struggled again last week when it was used in Beijing for the China Open. After being warned in his opening match he told the chair official: "I’m always very quick, I never get warnings. You gave me two in one set and it's impossible to play tennis like this."