Alcaraz is 29th after he won his first professional title in Umag last year, also on clay. Alcaraz's first professional win in the circuit was also in Rio de Janeiro two years ago.
The 18-year old said, "I felt like home today," before an enthusiastic Brazilian crowd.
Other matches saw Pablo Andujar of Spain beat Pablo Cuevas from Uruguay 7-6 (2), 7-5. Pedro Martinez, another Spaniard, defeated Shang Juncheng of China 6-3,6-4.
On Tuesday, three seeded players will compete in round-of-6 matches: Chile's Christian Garin (5), Italy's Lorenzo Sonego (6), and Spain's Albert Ramos Vinolas (8).
There are two top-10 players in the field. Italy's Matteo Berretini is ranked No. 6 and Norway's Casper Ruud who moved up to No. 8, after winning Sunday's Argentina Open.
