A 19-year-old Australian tourist knifed during an unprovoked attack at a McDonald’s near Penn Station was enjoying a meal with his friends when the suspect “calmly” walked up behind him and stabbed him twice — puncturing his lung, Manhattan prosecutors said Tuesday.

These chilling new details emerged as accused stabber Juan Mercedes, 25, pleaded not guilty on attempted murder charges for Monday’s stabbing, officials said.

The tourist was grabbing a bite with friends at the Eighth Ave. fast-food joint near W. 35th St. when Mercedes attacked him about 2 a.m., according to Manhattan Assistant District Attorney Kristen Tierney.

Surveillance footage shows Mercedes approach the suspect from behind before stabbing him once in the lower right torso and again in the lower right back, Tierney said.

“I got stabbed!”  the victim was heard shouting, according to a criminal complaint.

The victim suffered a punctured lung and was rushed to Bellevue Hospital, where he was still recuperating Tuesday.

Seconds after the unprovoked attack, Mercedes walked out of the McDonald’s and was quickly apprehended by police, who spotted him still holding the knife, Tierney said.

Mercedes didn’t know either the victim or his friends. There’s no indication that he had spoken to the victim before the attack, prosecutors said.

The suspect likewise has no connections to New York City, according to Tierney, who said Mercedes would not give investigators a straight answer regarding where he was currently living.

Mercedes’ last known address is in Port Charlotte, Fla, prosecutors said.

A judge ordered Mercedes held on $150,000 cash and $450,000 bond.

An email to the Legal Aid Society, who represented Mercedes at his arraignment, was not immediately returned.