Carlos Alcaraz pulled out of the Australian Open because of an injury.
The 19-year-old from Spain wrote on social media that he got hurt through a chance.
The first Grand Slam of the year takes place in January. Next Thursday is when the draw will take place.
The US Open winner moved to the top of the rankings. He became the youngest man in 50 years to finish a season at the top of the rankings.
The biggest single-season jump to No. 1 was his rise from 32 to 1.
He has had two significant setbacks in the last few months. In November, Alcaraz tore an abdominal muscle while competing at the Paris Masters. He was forced to withdraw from the finals.
He had a record of 57-13 and 5 singles titles.
"I'd worked so hard to get to my best level for Australia but unfortunately I won't be able to play at the first Grand Slam of the year," Alcaraz wrote on the social networking site.
He said that it was tough, but he had to be optimistic.
The seedings for the hard-court tournament will be changed because Alcaraz is out.
Since 1996, Americans Pete Sampras and Michael Chang have been the only players from the same country to top the rankings.
Alcaraz's departure removes a potential hurdle for Novak as he returns to Australia and tries to win it for a 10th time to claim his 22nd major championship overall. He was deported from the country because he wasn't up to date on his vaccinations. He is not inoculated but restrictions have been loosened.