The former Bayern and Werder Bremen player spoke for the first time on Friday about how he received the news, and said he had to react quickly to make sure he had made the cut.
“I was having lunch with my Freiburg teammates and saw I had three missed calls on my phone, so I was hoping no-one else had been called,” Petersen told a press conference while on duty with Germany.
“The next few days were different than planned. I was surprised but extremely excited to be involved in the squad.”
A young fan reporter at today's press conference: "Are you angry that you don't have a Panini sticker?" 😮— Germany (@DFB_Team_EN) May 25, 2018
Nils #Petersen: "Great question, but no. I'm just happy to be here, and maybe I'll be in the sticker book one day!" 😉#DieMannschaft #ZSMMN pic.twitter.com/RJ4Qk9A5nP
Petersen’s father is thrilled about his son’s achievement, too, but the Freiburg player opted to tell his girlfriend first.
“My father would have been the most excited, but it was my girlfriend that I told first as we had to change holiday plans,” Petersen said.
“He was so euphoric about my nomination. He probably would’ve announced it to the world straightaway, which is why I told my girlfriend first."
Petersen earned the call-up after scoring 15 goals in 32 league matches for Freiburg this season, helping to keep his club in the top flight for another year.
The top-scoring German forward in the 2017/2018 Bundesliga is uncapped at senior level, and was thankful to receive such unexpected news so late in his career.
“You always think about playing for Germany,” he said. “Twenty-nine years of life have brought me here and I’m very proud to be a part of the squad."
Watch: Take a look at Petersen's wondergoal against Borussia Dortmund!
Petersen could still be left out of the 23-man squad that will travel to Russia for next month’s tournament, but one thing in his favour is his incredible impact as a substitute.
No player in Bundesliga history has netted more times from the bench – 20 – than Petersen, and Löw cited his goalscoring record as a substitute – or a ‘Joker’ as Germans refer to supersubs – as one of the reasons he was considering him.
“I’m well aware of my nickname being the joker,” Petersen said.
“It’s probably part of the reason I’m in the squad, but I’m someone who tries to make a difference for the full 90 minutes.
“Sometimes a few minutes can change a game, and so it’s important not to be down should you be on the bench, but ready to take advantage of the moments if they come."