Mickelson, a perennial contender at Augusta National, withdrew from last week’s Texas Open during the third round with a pulled abdominal muscle.
It was merely a precautionary decision, because the three-times Masters champion was at Augusta National early this week for a practice round on his way to Houston.
“I felt great today,” Mickelson told Golf Channel. “I didn’t feel any pain or discomfort and didn’t even think about it.
“I’m surprised, because I was worried when it happened about the Masters but it healed a lot quicker (than expected).”
Mickelson ended the day equal 18th, with Haas and Hoffman shooting 65 to head a group of five players, including Matt Kuchar, by one stroke.
Five-time PGA Tour winner Haas, whose father Jay won the Houston Open 27 years ago at a different course, picked up five strokes in his final six holes.
Haas already has a spot in the Masters but Hoffman needs to win this week to earn a last-minute ticket.
Kuchar, who was tied for the lead midway through the final round at the Texas Open before fading to finish tied for fourth, bounced back quickly.
“I was 18 for 18 for greens in regulation,” said the six-times PGA Tour winner, who started on the back nine and picked up three birdies in his first six holes before adding three more in consecutive holes around the turn.
“I made a couple of nice putts mid-round to really get things going and missed a few coming in.”
Englishman Ian Poulter shot a 70 that included a two-stroke penalty at the par-five 13th, where a shot from a fairway bunker hit the lip and then struck his club.
American Dustin Johnson withdrew after shooting an 80 that included a quadruple bogey.
The Houston Open has attracted a quality field that includes 23 of the top 50 players in the world.
World number three Henrik Stenson is the top-ranked player, in the absence of Tiger Woods and Adam Scott.
Stenson, who shot 71, will become the world number one if he wins on Sunday.
(Reporting By Andrew Both in Cary, North Carolina; Editing by Peter Rutherford/Frank Pingue)