Tributes flooded in for Luca Scata, the 29-year-old Sicilian trainee who fired off two rounds after Amri opened fire on his partner Christian Movio, 36.
Senior officers described Amri as displaying an ice-cold cool as he pulled out a pistol after the officers had asked him to empty his backpack.
But Scata, baby-faced with a trendy beard, was just as quick. After Movio was hit by a bullet in the shoulder he fired twice in quick succession, sending the suspect to the ground.
Scata had only been in the police for nine months and was three months into a probationary period in Milan when he was put to the ultimate test.
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The more experienced Movio was recovering in a Milan hospital after a minor surgery.
He was well enough to receive visitors and paid tribute to his younger colleague in comments relayed by one of them.
"My partner's actions were exemplary, he reacted immediately when the pistol came out," he said.
"At a very young age, these extraordinary men have done a huge service to the entire community, by doing their duty.
"Italians can be proud of them, they have ensured a much happier Christmas."
Prime Minister Paolo Gentiloni echoed the praise as Scata's Facebook page was flooded with well-wishing messages from all over the world, with many demanding the government give him a medal.
At home in Sicily, his father Giuseppe Scata said he was just happy his son was alive. "He is a brave boy and he did his duty," he told reporters.
Anis Amri, a 24-year-old failed asylum seeker who arrived first in Italy before heading to Germany, is suspected of killing 12 and wounding dozens more on Monday by driving a truck into a Berlin Christmas market.
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