Novak Djokovic's shock exit from the Italian Open on Sunday could prove to be a decisive moment in the race for the world No. 1 spot, as the top-ranked Serb prepares for his French Open title defence with Jannik Sinner poised to leapfrog him.

Two days after accidentally being hit on the head by a water bottle while signing autographs in Rome, Djokovic crashed to his earliest defeat in the tournament he has won six times after a 6-2, 6-3 third-round thrashing by Alejandro Tabilo.

Djokovic will remain 1,090 points ahead of Sinner before the year's second Grand Slam begins on May 26, but the 36-year-old is on shaky ground as he will be defending 2,000 points and winning the title may not be enough to stay on top.

"It was a very unfortunate situation... that guy leaned over the fence, and the bottle dropped from his rucksack and landed on my head," Djokovic said, reflecting on the freak accident that hurt his chances to pull away from Sinner.

"It was unexpected. I wasn't even looking up. Then I felt a very strong hit in the head. That has really impacted me. After that, I got the medical care. Been through half-an-hour, an hour of nausea, dizziness, blood.

"I managed to sleep okay. I had headaches. Yesterday (Saturday) was fine, so I thought it's okay. Maybe it's okay. Maybe it's not."

With only six matches on clay, Djokovic will be slightly undercooked as he heads to Paris in search of a record 25th Grand Slam title.

Even if Djokovic goes on to win at Roland Garros, Sinner could rise to world No. 1 for the first time on June 10 by reaching the final.

The 22-year-old Sinner, who skipped Rome with a hip injury, will be defending only 45 points in Paris but faces a race against time to be fully fit.