Ireland's Paul Dunne and Scotland's Marc Warren shared the lead on six under in the Open de Espana.
Dunne, from Greystones, enjoyed a fast finish with three birdies and an eagle on the 18th in a back nine of 31.
"I find the greens quite tricky to read, especially on the front nine, so it was nice to get a couple of putts to go in on the back nine and climb up the leaderboard," Dunne, who won the British Masters last year, told Sky Sports.
Warren, whose best result in seven starts this season is a tie for 53rd, finished his 66 in style with five birdies in the last seven holes.
"I'm delighted with that start," Warren said. "I finished with three birdies in a row so it kind of makes it look a little bit better obviously. Overall, it was really solid tee to green.
"I putted really well today, my pace was really good. The putts that missed I didn't leave myself much to do and the good putts always looked like they had a chance of going in."
World number four Jon Rahm surprised himself with an opening 67 to lie just a shot off the lead in Madrid.
Rahm, who finished fourth in the Masters on Sunday, carded an eagle, four birdies and a solitary bogey at the Centro Nacional de Golf, where he practised during his amateur days.
The 23-year-old was part of an 11-way tie for third on five under par, which included Spanish amateur Victor Pastor.
"It feels great," Rahm told Sky Sports. "To be honest I would have taken anything under par given the fact that I played better than I expected, especially off the tee. My long game was amazing.
"The driver alone set up two birdies today on 13 when I hit it over the green and seven where I hit it to the front edge of the green. Things like that are obviously a bonus. Hopefully I can keep hitting it tee to green the same way and make a few more putts the rest of the week."
Asked how he was feeling following the trip across the Atlantic, Rahm added: "The morning is not as bad as the afternoon. When jet-lag kicks in after the round and adrenaline goes the afternoon is a little harder.
"I'm a lot better than I expected and hopefully it keeps going because sometimes jet-lag tricks you. It gives you a good day and the next day is horrible."
England's Aaron Rai shared the lead after covering the back nine in 30 with two birdies and two eagles, but eventually signed for a 67 after two bogeys and a birdie in his last five holes.