SAN ANTONIO, Texas, April 4, 2013 (AFP) - The US duo had a one-stroke lead over Ireland's Padraig Harrington and Americans Billy Horschel, Bryce Molder and Harris English, who all finished a windy day at TPC San Antonio on four-under 68.
A group of 10 players on 69 included England's Brian Davis, Australians Nathan Green, Alistair Presnell and Steven Bowditch, as well as Argentina's Andres Romero.
Tomasulo, making just his sixth start of the season on the PGA Tour, played in the last group of the day and despite some difficult conditions, he avoided big mistakes.
Three-time major champion Harrington said "mental fortitude" rather than great ball-striking was the key to his round.
He missed seven greens, but got up-and-down from three bunkers -- including a blast out at the par-five 14th that rolled within a foot of the cup and left him a tap-in for birdie.
A solid performance on the greens was marred by a three-putt from 34 feet at 18, where his first try whizzed past before he missed a four-footer coming back to finish with a bogey.
"That was a pity. Takes the shine off the day," he said. "It wasn't a difficult putt. As I got over it I stood off it once because the wind was gusting from my left, which it shouldn't have been," he said.
A gusty wind from the north dogged players much of the day and made it feel colder than the temperature of about 50 degrees Fahrenheit (10 Celsius).
"This morning, early on, it was a battle," Harrington said. "Last week back in Ireland it was snowing and I didn't feel this cold."
Even with his lapse at the last, Harrington had a share of the clubhouse lead with Horschel before Bettencourt and then Tomasulo completed their 67s.
Northern Ireland's Rory McIlroy, dislodged from the top of the world rankings by Tiger Woods last month and trying to sharpen up his game before next week's Masters, had four birdies and four bogeys in an even-par 72.
Leading first-round scores here on Thursday in the $6.2 million US PGA Tour Texas Open (par-72):
67 - Matt Bettencourt, Peter Tomasulo
68 - Padraig Harrington (IRL), Billy Horschel, Bryce Molder, Harris English
69 - Jason Gore, Brian Davis (ENG), Daniel Summerhays, Jim Furyk, Nathan Green (AUS), Andres Romero (ARG), Jeff Overton Ben Kohles, Alistair Presnell (AUS), Steven Bowditch (AUS)
70 - Peter Hanson (SWE), Martin Laird (SCO), Lee Janzen, William McGirt, Brad Fritsch (CAN), Brendon de Jonge (ZIM), DJ Trahan, Ian Poulter (ENG), Freddie Jacobson (SWE), Retief Goosen (RSA), Joe Durant, Matt Every, Shane Lowry (IRL)