The 30-year-old Hewitt has defied a spate of injuries and a free-falling world ranking to reach the fourth round of the Australian Open and a showdown with defending champion Novak Djokovic.
The two-time grand slam champion, who has slipped to 181 in the rankings, typified his fighting qualities with a resilient four-set win over big-serving Canadian Milos Raonic on Saturday to progress to the round of 16.
Nadal said he was emotional watching Hewitt, a former world number one, giving his all to get past Raonic, at 21, nine years his junior.
"I admire him a lot. I think he deserves the respect and admiration of all the people who love this sport" after so many operations, he said.
"I watched the match at my hotel and even though I have a good relationship with Raonic, his coach is from Spain... I really got emotional when Lleyton finished," Nadal added.
"He fell to the floor, he was really showing his emotions, so it's something fantastic at this stage after having a fantastic career to keep having this motivation."
"I wish him all the best for the rest of the season," Nadal said. "I hope his problems will be fine. He's one of the more charismatic players on tour.
"I always have admired him, his spirit of competition. Today he is an example to follow for a lot of people."