Rafael Alarcon, Bob Gaus and John Kernohan share second at 7-under, while Tim O'Neal, Richard Johnson, Tim Petrovic and Oscar Serna are a two shots back at minus-six.
Couch began on the back nine in Thursday's opening round at the Club Campestre and birdied his first three holes. He carded birdies at No. 15 and No. 17 to go out at 5-under 31.
Couch birdied his 10th, the par-5 1st, but then dropped a stroke with a bogey at the fifth. He took the lead in the tournament with a birdie at the eighth and then a 50-foot eagle putt at the ninth for his 64.
'I really don't know why but I like what is going on and I hope it continues,' said Couch, whose victory last week was his first on the Buy.Com Tour. 'I think that it's a little bit of a breakthrough last week finally winning, and I'm playing with a lot of confidence right now.'
Alarcon had a chance to match Couch at the top of the leaderboard but missed a four-foot eagle at his last, the ninth. He was flawless on Thursday when he collected seven birdies and no bogeys for his 65.
Alarcon has played in seven of the last eight Monterrey Opens with varying levels of success. Five times, Alarcon has missed the cut, but his two other finishes were as runner-up, including a playoff loss to PGA Tour player Stuart Appleby in 1994.
'The problem I have had in the past is the first nine holes,' said Alarcon, who played the first nine holes at two-under. 'Usually, I played in even-par or one or two-over and that has killed me because you have to make sure to get off to a good start to get good momentum going.'
Gaus also went bogey free on Thursday while Kernohan posted six birdies, an eagle and a bogey for his 65.
O'Neal was in good position to own the lead after he got to 7-under through 12 holes, but problems hit at the 13th. He went bogey, double-bogey, bogey over a three-hole span (the 13th-15th) that dropped him to minus-three.
O'Neal rebounded at the eighth when he hit his sand wedge from 104 yards 10 feet over the hole and then watched as it spun back and into the cup for an eagle. He birdied the last hole to finish with a 66.
Mike Brisky, Keith Clearwater, Brian Hull, Jason Caron, Jason Buha, Pablo Del Olmo, Mike Heinen and Deane Pappas share ninth after opening-round scores of 5-under 67.
Click here for full-field scores from the Buy.Com Monterrey Open