He will still have a shot at achieving the calendar Grand Slam when the US Open begins later this month, having already won titles at the Australian Open, Roland Garros and Wimbledon so far this year.
Nadal also gave his verdict on whether he thought the Serb could be the first man since Rod Laver in 1969 to achieve such a feat.
“He’s already won three, so when you win three, you can win four, without a doubt.
“He’s going to be playing on hard courts, probably his best surface, so why not? Of course it’s something difficult.
“[There are] going to be other guys that want to achieve the last Slam of the season. But of course he’s one of the clear favourites.”