Haas made five putts outside of 12 feet, including a 19-footer for eagle at the par-5 15th, a 29-foot birdie at the par-4 second and a 14-footer par save at the par-4 11th.
He'll face some stiff competition this week, including from Jordan Spieth, Jon Rahm and Justin Thomas. He leads five players at 5 under, a group that includes Tour winners Rickie Fowler, Billy Horschel, Chris Kirk and Bryson DeChambeau.
"I knew going out early this morning when it was cold the first few holes on the back nine can jump up and get you if you're not careful", Fowler told Golf Channel.
The other story of the week was the return of Tiger Woods, who despite driving difficulties, still shot 3-under par for the week and finished in a very respectable tie for 23rd place.
After starting the season with two missed cuts followed by a tie for 54th, Haas was not getting too carried away by his good start.
West Ham defender Arthur Masuaku apologises over red card for spitting
The Hammers went on to lose the game 2-0 and were eliminated from the tournament, with manager David Moyes livid at the player's actions.
"I'm going to do my best this year to three-peat and if I was fortunate enough to do that, I think I would join the "King", Arnold Palmer, as the only three-peat victor".
Hideki Matsuyama, a playoff victor over Fowler in 2016 and Webb Simpson previous year, had a 69 playing alongside his two sudden-death opponents. We're right back where we left off (before that)'.
One silver lining: He did birdie the par-3 16th hole twice - and with no bogeys.
In each of the last two years, Lefty has gone super low on Saturday (65 on both accounts), so maybe if he can string something good together on Friday, we'll be in for another treat over the weekend.
Two-time defending champion Hideki Matsuyama of Japan shot a 69 on Thursday and is five shots off Haas' lead. Palmer won the Phoenix Open three times in a row from 1961-63. Matsuyama has not finished worse than fourth in his four appearances at the TPC Scottsdale. In terms of total attendance, the traditional Super Bowl weekend date for the tournament results in the highest numbers on Tour, previous year reaching a record 655,434 for the week. That's one shot to the green and four while on it, if you're keeping track.