The Thai golfer carded a four-under par 68 on day two, enough to give him a one-shot lead over Australia's Scott Hend, Chilean Felipe Aguilar and American David Lipsky.
A further trio of players - Denmark's Anders Hansen, India's Rahil Gangjee and South Korea's Baek Seuk-hyun - are tied for fifth, two shots off the pace.
Pittayarat grabbed the headlines on the opening day of the European Tour event with a stunning nine-under round of 63.
And although he could not replicate his blemish-free performance one day later, he retained his advantage with a round that included five birdies, giving him an overall score of 13 under.
Hend stormed into a tie for second after hitting eight birdies in a seven-under 65. Of his birdies, four came from holes eight to 13 as he found his range at the Laguna National Golf and Country Club.
Aguilar and Lipsky are also locked at 12 under after shooting 67 and 68 respectively.
Hansen's impressive comeback from injury continued with a six-under 66 that took him to within two shots of the lead.
The three-time European Tour winner returned from wrist surgery last month and tied for fifth in last week's China Open.
He followed up a first-day 67 with another impressive effort, but was made to rue a three-putt bogey on his final hole, which cost him a chance of being tied for second.
Gangjee and Baek both shot 67 to remain in touching distance of Pittayarat.
Swede Kristoffer Broberg and Thailand's Arnond Vongvanij are tied for eighth at 10 under while a group of five players - including in-form Englishman Tommy Fleetwood - are a further shot back.
Defending champion Brett Rumford of Australia is in a tie for 24th at seven under, six shots off the pace.
Edoardo Molinari missed the cut by just one stroke, with the Italian the biggest casualty at the halfway point.