However, it opened with a 2-0 victory over Belgium " one of the favorites for the tournament " and then went on to secure top spot in Group E with a game to spare by beating Sweden 1-0.
That ensured it had the luxury of resting several players for its final match, a 1-0 defeat to Ireland, before taking on Spain at the Stade de France.
"Our strength is our humility," Chiellini said. "That's what helped us beat Belgium and Sweden. We have been a bit the surprise of these Euros, no one expected it.
"Now we're playing the champions of Europe ... On paper they are unbeatable. But I'm convinced that if we all play as we can, with all our qualities, giving our all, we can still achieve something important in this competition."
Chiellini is a stalwart in an Italy defense that kept Belgium's much-vaunted attack quiet and also shut out Zlatan Ibrahimovic's Sweden.
The spine of that defense has been playing together for years at both club and country level, with Chiellini, Andrea Barzagli, Leonardo Bonucci and veteran goalkeeper Gianluigi Buffon all teammates at Juventus.
"Our defense is obviously a point of strength of this squad, we've been playing together for so many years," Chiellini said. "We try to give the security to our teammates around us, most of whom are younger. We have two jobs: to do our job on the pitch and help build up the self-esteem."
Spain striker Alvaro Morata, who scored three goals in the group stage, spent the past two years at Juventus before Real Madrid exercised its option to buy him back this week.
"Alvaro has incredible skills and paradoxically still has a lot of margin to improve," Chiellini said. "He's grown a lot in the past two years ... I hope he's not going to do well on Monday, but I wish him the best in the future."