World number one Rafael Nadal described the timing of Andy Murray's withdrawal from the US Open as "strange".
Murray, the 2012 champion in New York, withdrew from the year's final grand slam on Saturday, citing a hip injury.
Nadal, who beat Dusan Lajovic in straight sets in the first round on Tuesday, questioned the timing of the Brit's decision.
"I saw him when I arrived here, and I was just saying hi to him. But I always thought that he was going to be playing if he was here practicing," the Spaniard said.
"It was a little bit strange that he retired just the morning after the draw was made. It was something that is a little bit strange and difficult to understand, but the worst thing is he is not healthy and I wish him a very fast recovery.
"Injuries are bad for everybody. I know better than all of them. So I wish him a fast and good recovery. That's the most important thing."
Murray's withdrawal left Nadal and Roger Federer on the same side of the draw, but had he pulled out earlier, the two greats could have met in the final.
Pressed on why Murray's decision was strange, Nadal said: "Because normally when you retire on -- was it Saturday morning? And the draw was made Friday? Normally you want to keep practicing, keep trying until the last moment.
"You don't retire Saturday morning. You retire Monday morning or Sunday afternoon, not Saturday morning. If not, you can do it before the draw.
"That's why I say it's strange. But of course he has his reason, and for sure the only news and the negative news was that he will not be playing here."
Nadal will face either Taro Daniel or Tommy Paul in the second round, with most matches postponed a day after persistent rain in New York on Tuesday.