Tau, who was absent during Sunday's 2-1 victory over Anderlecht, started from the bench against their top-flight counterparts.
Brugge looked for an early opener when Hans Vanaken played a through ball for Jan Vormer, who saw his attempt thwarted with a timeous challenge from Ibrahima Seck.
Vanaken then went on the attack himself and saw his header cleared off the goal-line on 31 minutes with the hosts living on the edge.
Brugge continued their fine start in the second half and duly took the lead through Mats Rits following a cross from substitute Charles De Ketelaere on 53 minutes.
Waregem drew level courtesy of Burundi captain Sadio Berahino six minutes later, as Bafana Bafana's Tau entered after 62 minutes for Denis Okereke.
The two sides pushed for the winner in the latter stages of the second half, but neither defence budged as the match ended all square.
The second leg is scheduled to take place on 5 February at the Regenboogstadion.