Lowry, who shot 6 under 65 in Round 1, is 3 under through 16 holes in Round 2 and holds a one-shot lead over Trey Mullinax at 9 under overall for the week.
Lowry, a European Tour victor in January at Abu Dhabi, stood on nine-under 133 after 36 holes at Harbour Town in Hilton Head, South Carolina.
The albatross fueled a 31 on the front, his second nine of the day, that more than compensated for consecutive bogeys to start the day and moved him to 7 under.
"It was a hard all day", said Johnson, who defied winds gusting to 30 miles per hour and made five birdies in his 67 to lead by a shot with Grillo from Rory Sabbatini (69) and CT Pan (65) on seven-under.
"It was hard all day", the world number one said.
But he responded with three birdies in a row, knocking in a six-footer at the 181-yard fourth and a five-footer at the par-five fifth before sinking a 20 footer at the sixth.
He knocked his drive 282 yards in ideal position, then landed a hybrid club into the cup from 254 yards away for only the second albatross at Harbour Town since 1983 and first since 2007.
Grillo closed his round perfectly too, with a hole-out from 37 yards away on the tricky ninth hole. "Perfect distance. flawless everything. Flawless wind. It was one of those that was good, but you never expect it to go in".
"At times it was really hard to judge the wind but I felt like I played really solid all day, gave myself a lot of opportunities". "I wanted to take my time and try to hit the best putt I could". "Hopefully, I can roll in a few putts over the weekend".
The World No. 1 is coming off a T-2 finish at the Masters and has two wins already this season with victories at the Saudi International and WGC-Mexico Championship. Italy's Francesco Molinari, who took a two-shot lead into the final round at Augusta National before being overtaken by Tiger Woods, was unlikely to make the projected cut of even par.
After an opening-round 71, Jordan Spieth was three under through 12 holes and in a tie for 29th when play was suspended.