Novak Djokovic is resigned to again missing marquee tournaments in the United States after confirming his stance on COVID-19 vaccinations remains unchanged.

The 21-time grand slam winner is set to sit out the Indian Wells Open and Miami Open after the US extended the requirement for all non-American nationals to be vaccinated.

Djokovic's participation at August's US Open may also be in doubt should those requirements, which are currently in place until April 10, continue to be extended.

But having also been forced to miss events in the likes of the US, Canada and Australia last year, Djokovic accepts he may have to do the same in 2023.

"I don't think there's anything official yet, so when it is we can speak about it," Djokovic told reporters on Thursday when asked about the US restrictions.

When told the rules have been certified by the US Transportation Security Administration, Djokovic replied: "I mean, if it is official then it is – what can I do? Nothing.

"You know my position, so it is what it is. I'm hoping [to play], but if I can't go, I can't go."

Djokovic, cleared to compete in Australia this year, was speaking after defeating Quentin Halys 7-6 (7-3) 7-6 (7-5) in the last 16 of the Adelaide International.

The 35-year-old, who beat Constant Lestienne in straight sets in his opening singles match of 2023, trailed 5-2 in the first set but recovered well and then edged the second set.

"It was a great performance from my opponent today and I want to congratulate him for a great fight," Djokovic said in his on-court interview. "He played like a top-10 opponent.

"He was serving big, hitting his spots and on a fast court like this it's tough to break, so two tie-breaks is a realistic score in today's match. I'm glad to overcome the challenge."

Djokovic will now face Denis Shapovalov, who saw off Roman Safiullin 6-4 6-3, for a place in the semi-finals.

"Denis is one of the most complete players out there," Djokovic said of his next opponent. "He has a very dynamic style of tennis.

"I haven't played him for a while so I'm looking forward to it. Every match from here gets tougher."

Sebastian Korda also advanced on Thursday with a 6-4 6-4 victory against Roberto Bautista-Agut, while Jannik Sinner beat Thanasi Kokkinakis 7-6 (7-2) 6-4.

At the Maharashtra Open in Pune, top seed Marin Cilic withdrew from his quarter-final with Tallon Griekspoor due to a knee injury.

World number 95 Griekspoor will now face Aslan Karatsev for a place in the final.