Both nations guaranteed themselves top-two finishes in the pool with dramatic last-minute winners in their last matches, and now they will do battle in Franceville to determine who will finish top.
Gabon only need a draw as they have a superior goal difference, and for Rohr the rewards for finishing in first place are multiple - including avoiding a possible quarter-final clash with 2010 runners-up Ghana.
Ghana are on course to finish top of Group D having won both their opening matches without conceding a goal.
Rohr said: "We want to avoid Ghana, so we would like to finish top. That would mean also playing our quarter-final in Libreville, in this wonderful L'Amitie stadium with its fantastic crowd.
"If we come in as runners-up we would have to go to Equatorial Guinea if we make it into the semi-finals."
He added to BBC Sport: "It will be a difficult game against Tunisia. They have two victories, too. If there's a draw we'd be first because we have a better goal difference. We play to win. So let's play a good game."
Gabon made it two wins from two on Saturday when, having seemingly lost two points after conceding a 90th-minute penalty equaliser, snatched a 3-2 victory over Morocco deep into stoppage time with a brilliant free-kick from substitute Bruno Zita Mbanangoye.
That secured their spot in the next phase and also sent through Tunisia, who had moved onto six points earlier in the day with a 90th-minute winner from another replacement Issam Jemaa against Niger.
Tunisia were the last country to win the African Nations Cup prior to Egypt's recent domination, beating Morocco 2-1 in the 2004 final.
Tunisia also defeated north African rivals Morocco 2-1 in their opening Group C match a week ago before seeing off Niger on Friday.
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