Lewandowski became the focal point of the Dortmund team that won two Bundesliga titles and reached a Champions League final.
Bayern Munich boss Carlo Ancelotti explains how he unsuccessfully tried to talk Philipp Lahm out of retirement.
Mainz brought a run of four games without a win to an end with a 2-0 victory over Augsburg at the Coface Arena to climb to ninth in the Bundesliga.
Mainz netted a goal in each half to fend off Augsburg and seal a home victory.
Due to injury and a crowded fixture list, Thomas Tuchel's Borussia Dortmund will need to get on without many familiar faces at bottom side Darmstadt.
Pep Guardiola has hailed Philipp Lahm as one of football's "best players ever" after the Bayern Munich defender announced he will retire.