The Scarlets and the Irish province are preparing to go head to head in the semi-finals on Saturday, with the Welsh outfit making the trip to the RDS Arena.
PRO14 champions Scarlets produced a fine performance to overcome La Rochelle in the quarter-finals but Pivac feels they will need to play even better to overcome Leinster this weekend.
He told Sky Sports: "They (Leinster) are unbeaten in the competition, and to do that home and away against the quality of their pool shows the strength of their squad.
"It'll be nice to have the opportunity to go a step further in this competition, and we certainly know it's going to be a massive task.
"Leinster are the form horse, and we are going to have to have probably our best performance so far as a group.
"It is just going to be one of those games where, from the first whistle to the last, we've got to be at our best."