Patrik Sjoland, Paul Lawrie, Philip Golding, Emanuele Canonica, Jarrod Moseley and Gary Orr are tied for fourth at 6-under 66. Jorge Berendt, Ricardo Gonzalez, Brett Rumford and Henrik Nystrom are one stroke further back at minus-5.
Poulter began his round at Olgiata Golf Club in routine fashion with pars on the opening two holes. Starting at the third, Poulter exploded to the top of the leaderboard with eight birdies over the next nine holes.
On the 12th tee, Poulter was minus-8 for his round and looking to become the first player on the European Tour to shoot a 59.
However, he parred three consecutive holes beginning with the 12th. With two par-5s in the final four holes the magic number was still within reach. Poulter made a move towards that mark when he stuck a 6-iron within 12 feet for eagle at the par-5 15th.
The Englishman followed that with a nice 4-iron which he landed within five feet of the hole at the par-3 16th. Poulter needed to finish birdie-birdie, but he could only manage a par on each hole.
'A 59 was definitely on,' Poulter said. 'This is the lowest score I have ever had but there were a few opportunities out there. I am so happy having walked off shooting 61 but it would have been nice to have shot 59.'
His tee shot on 17, the final par-5, was blocked right into the trees. After chipping out, his third landed within 18 feet of the hole. However, his birdie putt slid over the edge.
'I don't normally get nervous but I was more nervous on the 17th than when I won twice,' said Poulter. 'I want to win more this year than two years ago. I haven't had a win this season and I want to win. I feel I have to win every year.'
Hansen, like Poulter, started slowly with just two birdies over his first seven holes. He then raced up the leaderboard as he birdied four straight holes beginning at the eighth.
He struggled over the next two holes however, as he bogeyed each of them to fall back to minus-4. Hansen finished strong with birdies on the final four holes.
'I love this course. It was fun today,' said Hansen. 'I just tried to make as many birdies as I could and came out with ten. I have been struggling lately but played the last two weeks well. I played very aggressive out there. It is the end of the season and time to have some fun out there.'
Tunnicliff had a hot streak of his own to carry him into second place. He parred the first three holes then ran off five consecutive birdies starting at the fourth.
He later dropped a shot at 12, but recovered with a birdie at the next. Tunnicliff then closed with three birdies in the final four holes.
Jamie Spence, Peter Hanson, Gordon Brand, Jr., Maarten Lafeber, Mark Mouland, Niclas Fasth, Scott Gardiner and Christopher Hanell all stand at 4-under 68. Hanell is one of the two players that was unable to complete his round.
Padraig Harrington, who is chasing Retief Goosen for the Order of Merit title, opened with a 1-under 71. Goosen is playing in the season-ending Tour Championship on the PGA Tour this weekend.
Defending champion Gregory Havret struggled with an opening-round 75.
Full-field scores from the Italian Open