The Australian wild-card entry broke the 10th-ranked Monaco twice in the opening set and three times in the second at the indoor hard-court tournament. The loss makes it difficult for the Argentine to qualify for the season-ending ATP World Tour Finals in London.
Hewitt will face Ivan Dodig after the Croatian qualifier beat Philipp Kohlschreiber of Germany 6-2, 7-6 (6).
Fernando Verdasco beat Tommy Robredo 6-3, 6-4 in an all-Spanish match. He will play Marin Cilic of Croatia.
Also, David Goffin of Belgium beat Pablo Andujar of Spain 6-2, 6-2.