Rafael Nadal and Alexander Zverev moved through at the Mexican Open, while Nick Kyrgios was booed as his title defence came to an end. (More Tennis News)
A two-time champion of the event, top seed Nadal cruised past Spanish compatriot Pablo Andujar 6-3 6-2 on Tuesday.
In competitive action for the first time since the Australian Open, Nadal needed just 90 minutes to post his fourth victory in as many meetings with Andujar.
Zverev, the second seed at the ATP 500 tournament, survived an early battle before getting past Jason Jung 7-6 (8-6) 6-1.
Last year's runner-up, Zverev served 12 aces and broke four times in his victory.
Kyrgios beat Zverev in the final in 2019, but the Australian lasted just 31 minutes in the first round.
Ugo Humbert took the first set 6-3 when Kyrgios retired due to a wrist injury, the 24-year-old receiving some boos as he left the court.
Kyrgios was the only seed to fall as Felix Auger-Aliassime, John Isner, Grigor Dimitrov and Dusan Lajovic advanced at the hard-court tournament.
Taylor Fritz, Pedro Martinez and Kwon Soon-woo were also among the winners.
At the Chile Open in Santiago, seeds Juan Ignacio Londero, Federico Delbonis and Thiago Monteiro all got through their first-round matches.