The Azzurri fly-half became friends with Kolisi after spending two summers in Cape Town playing and studying, later joining Western Province Academy at 18. Kolisi, who also moved to Western Province after growing up in the impoverished Zwide township outside Port Elizabeth, is the first black player to captain the Springboks.
The two friends will be up against each other when Italy look to advance their claims on a Rugby World Cup quarter-finals place for the first time in the match at the Shizuoka Stadium Ecopa.
"He (Kolisi) is an inspiration for South Africa," said Allan, pictured above, whose side has just one win and 13 defeats against the Springboks. "Where he came from and where he is now, he can inspire the whole country.
"To know you don't have to come from a rich family to become something is an inspiration for South Africa as a country and he is a great leader for them."
While Allan did not play alongside Kolisi, he did appear in the same team as winger Cheslin Kolbe.
Kolbe, 25, spent last season helping Toulouse to the French title and his pace caused plenty of problems for New Zealand in their opening game.
Allan is a huge fan, saying: "He's an incredible player. I remember him from U19s - you'd give him the ball and he'd create something. It was that easy and he's doing the same at the professional level.
Wishing captain @Siya_Kolisi and his team of warriors all the very best for the Rugby World Cup. @Springboks, bring it home boys! ❤️🇿🇦 #StrongerTogether #Springboks #RWC2019 pic.twitter.com/kWHAB8VKQp— Ndlovu Youth Choir (@ChoirAfrica) September 21, 2019
"He's showing you don't have to be the biggest player to play rugby - you can be a small player but use your skills to overcome bigger opponents."
Italy go into the game top of Pool B having secured bonus-point wins against Namibia and Canada, with 26-year-old Allan scoring 20 points, including a try against Namibia.
"As a team we focused on this game as soon as the Canada match finished," he said.
"We know what the Springboks are capable of and the threat they hold, so we have the utmost respect for them. We know we have to show up at the weekend and give it our best."