The Durban side crossed for six tries to claim their third win out of four in the URC this season, but their victory came at a cost, with captain Andre Esterhuizen among four players injured on the night.
The Sharks led 17-7 at half-time, but the momentum swung dramatically after the break as Cardiff scored twice to take a 21-17 lead. Pietersen's men responded in style, however, hitting back with three unanswered tries despite their growing injury list to secure a bonus-point victory.
"It just shows the character in the group and the belief in the group," said Pietersen. "When we were under the pump, playing against the wind, the old Sharks could have easily lost 50-17, but we stuck in there.
"We lost Andre early in the game. We lost Phatu [Ganyane] early in the first half, and then obviously we lost Jurenzo [Julius] at half-time. And then the second half started with Jason Jenkins also going off.
"The beautiful thing about these Sharks warriors is that they found a way to get on with it and pull closer together," Pietersen said.
"You must give massive credit to the players for sticking in the fight and finishing the game off like that."
The result gives the Sharks five valuable log points as they continue their European tour - although another tough challenge lies in wait at Munster next weekend. Pietersen said the Sharks would likely need to fly in some replacements ahead of the game.
"We're happy with the result and over the moon to get five points in Cardiff," he added. "But we're also keeping our feet on the ground, as we know we've got a massive game against Munster in Limerick.
"When we have the update on injuries, we'll probably fly some guys in to help us finish off the tour."