Robredo, who made his professional debut in Barcelona in 1999 and lifted the title in 2004, had announced he would retire after playing at his home tournament this week.
The 39-year-old won 12 ATP titles and knocked Roger Federer out of the US Open in 2013. He reached seven grand slam quarterfinals.
Zapata Miralles, ranked 119th in the world, proved too strong for his fellow Spaniard, winning in just over an hour.
Italy's Lorenzo Musetti continued his excellent start to the clay-court season by beating Argentinian Sebastian Baez 7-5, 7-5, to reach the second round.
Musetti defeated world No 9 Felix Auger-Aliassime in Monte-Carlo last week.
American Brandon Nakashima also progressed. He beat Spanish qualifier Nicolas Alvarez Varona 7-6 (8/6), 6-3 and will face second seed Casper Ruud in round two.