I think Novak might be somebody who needs energy from a crowd to operate at his best. He has seemed to lack fitness/energy during some of these matches, but he was very strong physically in the 5th set against Tsitsipas at Roland Garros, where there were about 1000 people in attendance. It looked like he was fresh enough to go on for a couple more hours. He also looked energetic in his match against Schwartzman in Rome, where there was some crowd.
He's not as reliant on crowd energy as somebody like Monfils, who can't even win a set in these empty venues, but I do think it makes a difference for him in matches against top players. That isn't to say he would've won these matches with a crowd, but I don't think he has an endurance problem.