Andy Murray's impending retirement was a decision he had to take because he is "suffering", says long-time rival Rafael Nadal.
The 31-year-old Briton announced on Friday that he will quit this year, and next week's Australian Open could be the final tournament of his career.
"When you are going on court without a clear goal because you cannot move well and you have pain, then it is the time to take a decision," Nadal said.
"He will be a big loss for tennis."
Nadal, 32, knows more than most players what it is like to battle injury having had a catalogue of serious problems over the years, with his knees and wrists in particular.
But the Spaniard, a 17-time Grand Slam singles champion, says he has never "arrived" at the point of feeling he had to quit the sport.
"I always had the feeling that we'll fix it," said the world number two, who begins his Australian Open campaign against home wildcard James Duckworth on Monday.
"But, of course, there are periods of time that you don't see the light. It is tough.
"I know it is hard mentally. It is tough when you have one thing, then another thing.
"Andy has probably been fighting to keep going for a long time. If he doesn't feel that the injury can become better, he has probably done the right thing for his mental health."
Scotland's Murray first met Nadal, who has won 17 Grand Slam titles, when they were teenagers and played against each other at junior tournaments.
The pair have met 24 times as seniors, with the Majorcan winning 17 of the contests.
"I always had good relationship with him," said Nadal. "We shared moments in my academy. We shared courts in the most important stadiums in the world, competing for the most important things. That's impossible to forget.
"So all the best to him. We will miss him. But today is him. Tomorrow another one. We are not 20 any more. Our generation, everyone is more than 30 - these kind of things happen.
"He will be a very important loss for us, for the world of tennis, for the tour, for the fans. But that's life. It seems like he had not a very long career because today players are playing that long. But he's 31 - 10 years ago, if he retired at 31, we would say he had a great and very long career."