The top-seeded Nalbandian broke serve in the fifth game and held on to easily win the first set. In the second set, Nalbandian took two service breaks to Lapentti's one to take the match in 1 hour, 14 minutes.
"I started out with a strong match," said Nalbandian in his first appearance before a home crowd since Argentina's loss to Spain in November's Davis Cup finals. "After so much time not playing on clay, I adapted quickly. This surface is always a little more difficult for me.
"It's a pain that will never go away," the No. 10th-ranked player Nalbandian said.
Nalbandian said he would join Argentina in March to play the Netherlands in the first round of this year's Davis Cup tournament.
Heavy rains forced officials to cancel two of the six matches scheduled Monday and the wet weather made the clay court at the Buenos Aires Lawn Tennis Club even slower than usual.
In the closest match of the day, Argentine Brian Dabul beat Frenchman Nicolas Devilder 5-7, 7-5, 6-4.