The only problem was, the "fan" was 21-year-old Argentinean striker Sebastian Driussi and Messi completely failed to recognise him.
Messi is tipped to return from Moscow to Barcelona imminently.
The Barcelona star obliged, thinking he was just another fan, not realising he was a fellow professional.
"I know him and feel bad for not having recognised him at the time", Messi told Fox Sports Argentina.
Stanislav Cherchesov's side made it hard for Argentina, who handed debuts to Giovani Lo Celso and German Pezzella from the start, by pressing high in the opening stages, with Messi unable to get on the end of a bright exchange with Eduardo Salvio following a surge down the right.More news: Hasbro Makes Bid For Rival Toymaker Mattel
Messi was in action for Argentina in Saturday's 1-0 win over 2018 World Cup hosts Russian Federation in Moscow but will return to his base in Barcelona, Spain ahead of his club's Spanish La Liga fixture next weekend against Leganes.
He added: "When I saw him in the lobby of the hotel I was thinking of something else and I did not realise".
Driussi bumped into his compatriots in St.Petersburg, where he plays for Zenit when he asked for the photograph, which he then shared to his personal Instagram page, tagging Messi in the picture's caption.
The national side have become particularly reliant on Messi since his U-turn on retirement from global football, and needed his goals against Ecuador to ensure qualification for next year's tournament.