The two footballing icons spent a memorable four years together at Barcelona between 2008 and 2012, winning trophies for fun.
Messi played the best football of his legendary career under Guardiola's rule and the two appear to still be close friends to this very day.
Guardiola has always maintained that the diminutive Argentine is the greatest player in the sport's history.
Like the rest of us, the City boss just enjoys watching Messi weave his magic on the pitch and that was certainly the case in 2015.
In the early stages of that year, Guardiola returned to Barcelona to attend the Catalan side's Champions League match versus City.
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The Spaniard was managing Bayern Munich at the time, the team who the Blaugrana destroyed in the semi-finals of the competition a short while later.
And while he was watching his beloved Barcelona versus City, Guardiola witnessed Messi produce one of the greatest nutmegs in history.
Poor ol' James Milner was on the receiving end of the delicious skill move and after the Englishman was left on the turf, the cameras panned to Guardiola in the stands.
He was speechless and had to put his head in his hands...
Video: Messi's nutmeg stuns Guardiola
Footage of that nutmeg needs to be hung in the Louvre.
Messi didn't score versus City at Camp Nou in 2015, but he still put on a masterclass to help the team secure a 1-0 victory and passage to the quarter-final stage.
When Guardiola returned to Barcelona's home stadium to lead Bayern in the first leg of the semi-final that year, he certainly wasn't a happy man.
Messi scored twice and assisted Neymar's goal in a comprehensive 3-0 victory for the Spanish outfit.
Will Guardiola be subjected to more pain at the hands of the Argentine on Tuesday? You wouldn't put it past the six-time Ballon d'Or winner, despite the fact he missed PSG's last two games through injury.