Pienaar delivered a man-of-the-match display in the defeat of Edinburgh to earn Ulster their place in the final and has contributed 71 points during this season's European campaign.
Only Leinster now stand between Pienaar's side and the European title but the Ulstermen have already lost twice to their RaboDirect PRO12 rivals in the league this season.
And after finishing the domestic campaign with a run of three straight defeats to Connacht, Leinster and Munster, Pienaar, 28, insists Ulster will need to repeat their semi-final showing to stand any chance of overhauling Leinster.
"Leinster are a top-class side. The challenge could not be greater," he said.
"We showed our status as a solid defensive unit in the last two games and scrambled well, but apart from the first 20 minutes against Munster we have not had our hands on much ball.
"We can't really afford to do that against Leinster.
"They have world class players and dangerous backs so it would be a mistake to let them have the ball for 80 minutes, which is more or less what happened against Edinburgh."
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