ORLANDO, 20 Mar 2015 (AFP) - Overnight leader Hoffmann remained red-hot, nabbing nine birdies in a seven-under par 65 at Bay Hill for a 36-hole total of 13-under 131.
He had a three-shot lead over Sweden's world number three Henrik Stenson, defending champion Matt Every and Harris English -- who all carded six-under 66 for 134.
Ben Martin was alone in fifth after a 67 for 135, while McIlroy headed a group of four players on eight-under 136.
McIlroy had seven birdies in his six-under 66, including five on the trot on the inward run of a round that he opened with a birdie at the 10th.
He added another birdie at the par-five 16th, but although he played his first nine holes without a bogey, he had to do some scrambling -- finding three fairway bunkers.
McIlroy caught fire with an 18-foot birdie at the par-three second hole. He proceeded to birdie the next four, making putts of six, seven, two and 12 feet.
He gave back a shot at the eighth, where he was plugged in a bunker, but was plenty pleased with his first round under 70 on the US tour this season.
"I probably wasn't quite as consistent tee to green as I was (on Thursday), but the putter is a great equalizer," said McIlroy, who opened 2015 on the European tour with a runner-up finish and a win, but missed the cut in his first US start at the Honda Classic and also struggled at the WGC Cadillac Championship.
"Felt like I was pretty tidy, and the short game was good," added McIlroy, who needed just 25 putts. "Could have hit a few better shots, but there was a lot of good in there."
McIlroy is under the microscope with the Masters looming.
He could complete a career grand slam with a victory in the first major of the year, and also win his third major in a row after his victories in the British Open and PGA Championship last year.
"I think each and every day I'm feeling a little more comfortable, especially on the greens," McIlroy said.
Of his birdie burst, he added: "It feels pretty easy at the time. You are just trying to keep it going. You can sort of feel the momentum building. It would have been nice to finish the round off a little better, but that was still a good score and it sets me up well for the weekend."
He still has a lot of catching up to do, with Hoffmann rolling in pursuit of his first US PGA Tour title.
"I'm behind my schedule, I would have liked to have won a few times by now," said Hoffmann, who is in his third year on the tour.
Hoffmann's six birdies on the front nine included four in a row, and the self-described adrenaline junkie said his aim would be to keep up the torrid pace.
"I'm trying to birdie every hole out there," he said.
"Just hitting great putts and good shots into the greens really helps. It's nice ot hit some greens for a change. This year hasn't been that great, and it's a good change."
Leading second-round scores on Friday at the $6.3 million US PGA Tour Arnold Palmer Invitational (USA unless noted, par-72):
131 - Morgan Hoffmann 66-65
134 - Matt Every 68-66, Henrik Stenson (SWE) 68-66, Harris English 68-66
135 - Ben Martin 68-67
136 - Danny Lee (NZL) 72-64, David Lingmerth (SWE) 69-67, Rory McIlroy (NIR) 70-66, Matt Jones (AUS) 71-65
137 - Ian Poulter (ENG) 67-70, Louis Oosthuizen (RSA) 69-68, Sean O'Hair 69-68, Brooks Koepka 71-66, Kevin Na 67-70, Blayne Barber 72-65
138 - John Peterson 67-71, Ernie Els (RSA) 71-67, Brendan Steele 71-67, George McNeill 69-69, Jason Kokrak 67-71, Webb Simpson 69-69, Keegan Bradley 68-70
139 - Padraig Harrington (IRL) 68-71, Erik Compton 70-69, Charles Howell 71-68, Steve Wheatcroft 70-69, Ryo Ishikawa (JPN) 70-69, Billy Horschel 68-71