Ramsay had five birdies to go with his eagle against one bogey for a two-stroke lead over Ron Fisher, who shot 4-under 67 despite a triple bogey 7 at the eighth hole.
Stephan Gallacher and Gregory Havret are tied for third at 3-under.
Sergio Garcia, who won last week's European Tour event, is in a group tied at 1-under. Garcia's second nine Thursday included an eagle, three birdies, three bogeys, a double bogey and a par.
Ramsay, playing in Thursday's first group, was much more steady, despite windy conditions at the Club de Golf Valderrama course.
He was 4-under after 10 holes and then made a putt for eagle at the 547-yard, par-5 11th. He added a chip-in birdie at No. 15 but gave a stroke back at the par-5 17th.
"I worked out a good game plan with my caddie about how to play it in the wind and it's always nice to have a 65 around a classic course like this because it's had the Ryder Cup and so many great champions," Ramsay said.