Indian Wells organisers said on Tuesday that world number two Novak Djokovic had been placed in the tournament's draw but it was unclear if the Serbian would be permitted to enter the United States.
The 20-time Grand Slam winner has refused to obtain a COVID-19 vaccine and previously said he was prepared to miss tournaments as a consequence.
"Novak Djokovic is on the tournament entry list, and therefore is placed into the draw today. We are currently in communication with his team," Indian Wells organisers wrote on Twitter.
"However, it has not been determined if he will participate in the event by getting CDC (US Centers for Disease Control and Prevention) approval to enter the country."
The Indian Wells men's draw on Tuesday showed he had a first-round bye.
The CDC website as of March 3 indicated non-US citizens who are not immigrants must show proof of vaccination against COVID-19 in order to travel by air to the United States.
Djokovic was unable to compete at the Australian Open after immigration authorities detained him on January 6, prompting a legal rollercoaster ride over the country's COVID-19 entry rules that led to the cancellation of his visa later that month.
The California tournament, often referred to as tennis' "fifth major," is expected to welcome a full house after it was cancelled in 2020 and moved to October last year due to the COVID-19 pandemic.
The women's first round begins Wednesday while the men's starts Thursday.
With former world number one Roger Federer out of the tournament as he recovers from knee surgery and Djokovic previously presumed absent, Spaniard Rafa Nadal was widely favoured to pick up his fourth Indian Wells title and a men's record-extending 22nd Grand Slam title at the French Open after winning the Australian Open in January.
Djokovic most recently competed in Dubai in February, where Czech Jiri Vesely stunned him in the quarter-finals.
The loss meant that he would lose his world number one ranking to Russian Daniil Medvedev, who claimed his maiden major title in Flushing Meadows in September and is the top seed at Indian Wells.
France loosened COVID-19 restrictions earlier this month, with vaccine passports no longer mandatory as of March 14, opening the door for the 34-year-old to mount a title defence at Roland Garros beginning May.
