Jannik Sinner will return to the ATP Tour from his doping ban at next month's Rome Masters as world number one after the Italian's closest challenger Alexander Zverev crashed to an early defeat in Monte Carlo on Tuesday.

Sinner, 23, accepted a three-month ban in February following an agreement with the World Anti-Doping Agency, which earlier challenged an independent tribunal's decision to clear him of any wrongdoing after he failed two drug tests.

Sinner's absence from the circuit had opened the door for Zverev to replace him at the top of the rankings but the German failed to get beyond the quarter-final stage at any of the six tournaments he played since the Australian Open in January.

The world number two lost 2-6 6-3 7-5 to Sinner's compatriot Matteo Berrettini in the Monte Carlo second round.

"I've not won many matches. So it's been the worst period since my injury," Zverev said, who will have to wait till later in the European claycourt swing for another chance to displace Sinner.

"I played a great first set, and once I got broken in the second set, I played 10 levels down. My ball is much slower. I stop hitting the ball.

"The same story the last few months. Nothing changes. So it's me who lost the match, once again ... I thought my level was terrible, but that's just my opinion."

Defending French Open champion and world number three Carlos Alcaraz, who also squandered the chance to reclaim top spot before Sinner's ban ends on May 4, begins his Monte Carlo campaign against Francisco Cerundolo later on Wednesday.