Jannik Sinner struck first in the battle to end the French Open as world number one.
The 22-year-old Italian, who won his first grand slam at the Australian Open in January, is currently ahead of Novak Djokovic in the live rankings.
Sinner cruised into the second round at Roland Garros, despite injury concerns going into the tournament, with a 6-3 6-3 6-4 victory over American Christopher Eubanks.
Djokovic, who returned to the top of the rankings in September, now needs to at least reach the final to have any chance of staying there.
The 37-year-old Serbian, who said he has "low expectations and high hopes" of defending his title in Paris, faces French wild card Pierre-Hugues Herbert in his first-round match on Tuesday.
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