Bristol start the tie leading 28-13 after a dominant display at Castle Park last week.
But Knights are staying positive.
“Of course we can still win the tie. There’d be no point in us going down there if we didn’t think that,” said director of rugby Clive Griffiths.
“We’ll be giving it our best shot but if we do fall short over the two legs then I want us to be able to hold our heads up high at the end. I don’t want to go out with a whimper.”
It is a view shared by strong-running centre Will Hurrell, who has scored tries in all three play-off games.
“We said before the start of the first leg at Castle Park that the tie wouldn’t be won or lost on the night,” he said.
“Just as we did when we took a 13-point lead into the second leg in the semi-final against Yorkshire Carnegie, we’ll treat tonight’s game as though we are starting at 0-0.
“They’ll obviously fancy their chances but if it’s close going into the final quarter in the back of their minds they’ll remember that we beat them at home in the league.
“Thanks to the fitness work we’ve done with Dougie (Flockhart) we always go into a game knowing that we are capable of coming from behind and nicking a result at the end.”
Tonight’s game will mark the final game in a Doncaster shirt for first-team regulars Ollie Stedman and Jon Phelan.
Stedman comes into the starting XV at the expense of Latu Makaafi, in the only change to the side that lost in the first leg.
A sell-out crowd of 16,000 will watch tonight’s match.