Haas, 33 and twice a semi-finalist in Melbourne, has seen his career derailed by injury in recent years, but showed flashes of the play which saw him rise to number two in the rankings 10 years ago.
Two early breaks saw Nadal surge ahead and although Haas clawed one back, the Spaniard overcame some nervy moments to take the opening set in 54 minutes.
The second was straightforward with breaks at the start and the end, but Haas put up stronger resistance in the third by claiming a 3-1 lead.
Nadal hit back to level at 3-3 and then claimed the match when Haas missed a forehand at break point in the 10th game.
Afterwards, Nadal was quick to pay tribute to his opponent, saying: "He's a fantastic player. He's had a lot of injuries the last couple of years but it's always great to play Tommy.
"He's thinking about retiring but I thought he played well this afternoon [Wednesday]."
Nadal has had his own injury worries leading into the season's first grand slam, injuring his right knee while sitting on a chair on Sunday.
But although it was again heavily taped, the Mallorcan revealed the joint is on the mend.
"It is much better and I am so happy the knee is improving and I am able to play without pain," he said.
Reflecting on a good work out, he added: "I don't know if it was the perfect match but I won in three sets.
"It was positive but not that demanding (physically). We didn't play for four or five hours."