Rafael Nadal described Novak Djokovic's absence from the US Open as "very sad" after the Serbian revealed he would not compete at Flushing Meadows.
Djokovic announced he would not be making the trip to New York on Thursday, due to visitors to the United States being required to display proof of vaccination against COVID-19.
The US Open will represent the second grand slam Djokovic has missed this year, after his unvaccinated status saw him deported from Melbourne ahead of the Australian Open in January.
Nadal is one of the most likely players to benefit from Djokovic's absence as he bids to win a 23rd grand slam title, but the Spaniard expressed sympathy for his rival as he prepared to begin his US Open campaign against Rinky Hijikata next week.
"From my personal perspective, it's very sad news," Nadal said. "It's always a shame when the best players in the world are not able to play a tournament because of injuries or because of different reasons.
"It's tough for the fans, tough for the tournament. In my opinion, tough for the players, too, because we want to have the best field possible."
Nadal's clash with Hijikata will represent his first outing at Flushing Meadows since he beat Daniil Medvedev to win the 2019 title, and injuries have caused him to miss four of the last 10 editions of the tournament.
Roger Federer, another of Nadal's great rivals, will also miss the competition after undergoing three knee operations in two years, but the Spaniard believes the major championships can cope with the absences of tennis' aging stars.
"The world of tennis [will] keep going, even if it is not good news for everyone," he added. "The world continues and the tennis will continue after me, after Novak, after Roger.
"Without a doubt, Novak is one of the most important players of the last 20 years, [in] the history of our sport. In a personal way, I feel sorry for him that he's not able to travel here."
Meanwhile, Federer is hoping to return to the court for September's Laver Cup, where he will join Nadal and Djokovic in a star-studded Team Europe line-up, and the Swiss maestro's old foe is desperate to see him make his long-awaited comeback.
"I'm super excited to see Roger again on court, of course," Nadal said. "I hope he's healthy enough to make that happen.
"It has been a long period of time without Roger on court so I really hope that he can manage to be back.
"The most important thing at this stage of his career is the health and the happiness, no? If he's healthy enough and happy enough to be back on the tennis tour, it will be amazing.
"If not, we can say thanks to him for everything that he achieved. I am confident that we will keep having Roger for a while."
