Ruffels, who has been competing on the PGA Tour's third-tier Latinoamerica tour and is playing his first PGA Tour event since the Memorial Tournament last June, was considered one of the world’s best amateurs before turning professional in January 2016 and is slowly finding his feet in the paid ranks.
Chalmers is playing the Arnold Palmer Invitational for the first time in three years and his strong start represents a turnaround in fortunes having missed the cut in his previous three trips to Bay Hill.
Defending champion Jason Day is one shot further back at 2 under and three from the lead along with Victorian Aaron Baddeley.
Day was happy enough with his start having played in cold morning conditions.
“It was good,” the Queenslander said.
“I think it was tough to gauge how far the ball was going this morning. We worked it out about 15 or 20 yards. As the day goes on, you're, it gets a little bit warmer so the yardage starts to creep back up.
“You start hitting it a little bit further. But it's hard. You'll be standing over it and you sit there thinking, this club should be going further. But it's hard to commit to it sometimes because it's going so short in the cold weather and wind. But once again, I see the lead is at 5-under or so, I'm on 2, so it's a good start.”
GOLFING WORLD: GOLF LOVE - OLLIE SCHNEIDERJANS
Of the other Australians in the field, Marc Leishman is at 1-under and in a share of 20th, former winner Rod Pampling is 1-over and T46th, Cameron Smith sits T58th and major champion Geoff Ogilvy is 4-over and T92nd.
Steven Bowditch and recently crowned World No. 1 amateur Curtis Luck each share 110th position after rounds of 77.
Argentina's Grillo and Englishman Fitzpatrick are one clear of Americans Lucas Glover and Charley Hoffman and Englishman Paul Casey who each fired 4-under 68.
Fitzpatrick, like Ruffels, is playing on an invite this week and, at this stage, does not have PGA Tour status but it is something he is keen to pursue.
“Really important for me,” said Fitzpatrick when asked about the significance of this week. “It's something that really you're aiming for a win over here would be fantastic and I'm just trying to get my card right now and obviously that's significant.
“I would love to play more over here as well as keep my place in Europe, but it's something I need to sort of figure out, hopefully, if I get an opportunity I'll plan it properly.”
Grillo has already won on the PGA Tour but this is a golf course where he has performed well previously (he contended last year before finishing 17th on debut) and he was keen to get back.
“Definitely it's a golf course that I loved last year, I was looking forward to coming back here,” said Grillo.
“I played up to my expectations today. It was a tough time and I was able to turn the start around and I'm proud that I have."
DENNISE HUTTON: FOLLOW-THROUGH THROW