Union, enjoying their best ever season, are in fourth place on 56 points, as many as Freiburg, who are fifth on goal difference. RB Leipzig are third on 57.
The top four teams qualify for next season's lucrative Champions League group stage and neither Union nor Freiburg have ever reached that stage.
For Christian Streich's Freiburg, the bitter memories of last season's narrow failure to finish fourth following a 4-1 home loss to Union on the penultimate matchday are still fresh.
Another defeat in their final game saw them finish sixth, a spot behind Union.
They will have their work cut out on Saturday with the Berliners undefeated at home for 21 consecutive Bundesliga matches with their last loss more than 14 months ago.
Streich is likely to have Roland Sallai (25), Ritsu Doan (24) and Philipp Lienhart (26) back in the squad.
A few spots further up the table, league leaders Bayern Munich will look to protect their one-point advantage over second-placed Borussia Dortmund with a victory over relegation-threatened Schalke.
While the Bavarians are dealing with widespread speculation over the future of Bayern midfielder Thomas Mueller, who has been benched since the arrival of coach Thomas Tuchel, Schalke are brimming with confidence.
The Royal Blues snatched a last-minute win over Mainz last week to drag themselves out of the relegation spots for the first time since the start of the season.
Schalke are in 15th place, two points above the relegation play-off spot.
Ironically it is bitter Ruhr rivals Dortmund who will benefit most if Schalke upset Bayern.
They could return to the summit and have a shot at their first league title in 11 years if they beat Borussia Moenchengladbach on Saturday.
Even Dortmund's mayor pitched in this week, saying he would invite the entire Schalke team to City Hall "to inscribe themselves in Dortmund's Golden Book," should they beat Bayern and help Dortmund become champions.
"We are going to Munich in order to win for Schalke and for the fans," Schalke's Rodrigo Zalazar told the Munich Merkur newspaper. "Together with the team I want to make sure we stay up. Anything else does not concern me."
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