Romo carded the 2-under-par score Thursday at Silverado Country Club in Napa, Calif. Australian Adam Scott and American Andrew Landry were tied for the lead after shooting 7-under-par 65s in the opening round. World No. 9 Francesco Molinari was tied with American Matthew NeSmith for third place.
"If you are going to go do something, you want to do it well," Romo told reporters. "It's embarrassing to stink, and I've stunk a lot. I'm just trying to continue to improve and get better.
"It's the same thing I did in football. You start from the beginning and you're not very good. Then all of the sudden you make this leap somewhere."
Thomas -- the No. 6 ranked player in the world -- is tied for 51st after shooting a first round 71. Mickelson was 3-over-par in the first round. Romo had five birdies and three bogeys on the day.
Romo tees off at 1:25 p.m. EDT Friday in Napa. He hits off tee No. 1 with Beau Hossler and Michael Gellerman. Scott tees off with Hideki Matsuyama and Sebastian Munoz at 12:45 p.m. Landry tees off with Adam Long and Kevin Na at 7:40 a.m.
Brendan Steele won the 2018 Safeway Open. Mickelson finished tied for third at last year's tournament.
Romo -- the winner of the 2017 and 2018 American Century Championship -- had a previous career-low round of 74 entering the 2019 Safeway Open. He has yet to make the cut in three PGA Tour tournament appearances.