World No. 4 Jon Rahm surprised himself with an opening 67 in the Open de Espana to lie just a shot off the lead in Madrid.
Rahm, who finished fourth in the Masters Sunday, carded an eagle, four birdies and a solitary bogey at the Centro Nacional de Golf, where he practiced during his amateur days.
"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 after 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."
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."
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 on his last five holes.