Three days before his 36th birthday Gronberg returned a five under par 67 on the Mirasol course in Palm Beach Gardens and shared the early clubhouse lead with tournament favourite David Toms.
Harrington, playing with Toms, was much the faster out of the blocks with two birdies in the first three holes of the inward half.
Then came a double bogey six on the 16th, however, and he did well to avoid another on the 18th, sinking a 10-foot bogey putt after driving into water and hitting his third into a greenside bunker.
Although he added further birdies at the fifth and sixth, the two par fives on the front nine, Toms completed a run of five successive birdies on the same two holes.
“There are a lot of expectations with defending,” said Harrington. “Definitely the easiest way to win an event is to be playing well and to be under the radar. You go defend a tournament, you’re never under the radar and it does put a lot more pressure on you.
“It doesn’t happen too often, so you’re not as familiar with it. The only thing to make it as good as last year is win again this year - but it’s still a very nice problem to have.”
Toms is the only world top 10 player in the field, already has a win to his name this season and last Sunday was joint runner-up to Tiger Woods at the Ford Championship in Miami.
England’s Greg Owen was on the same mark as harrington following a hat-trick of birdies from the 12th late in the day.
Earlier Ian Poulter and Brian Davis could do no better than 75 and 76 respectively, while Luke Donald and Lee Westwood were level par and two over entering the closing stretch - Donald after a double-bogey six on the 13th.
After finishing only 160th on the US Tour last season Gronberg survived the qualifying school in December by the skin of his teeth, but so far this year has made only one halfway cut in four starts.
Winner of the European Open by a massive 10 shots at the K Club near Dublin eight years ago, he followed birdies at the first, fifth and sixth by hitting his tee shot to five feet on the short eighth and then almost holed a bunker shot at the 588-yard 12th.
He did bogey the 16th, but was on in two on the 556-yard next and two-putted for his sixth birdie.
Poulter, trying to climb from 65th in the world back into the top 50 in time for the Masters next month, double-bogeyed his very first hole and had another on the 16th, his seventh. He did make four birdies, but there were also three bogeys in an erratic display.
After the promise of a seventh place finish at Pebble Beach last month Davis is now in danger of a third successive missed cut, the most damage coming when he double-bogeyed the short eighth.