Having taken sole leadership of the league in the last match before the Christmas break, Hansi Flick's team will look to build a sizeable lead before the season reaches its business end.
The Polish striker's goals helped the Bavarians to overturn a deficit that had been inflicted by Patrick Schick's early goal.
Lewandowski scored the winning goal in second-half stoppage time at the BayArena to break the hearts of Leverkusen fans.
Mainz have won only one Bundesliga game this season, having played 13 matches so far. They are second from bottom, with only Schalke below them in the Bundesliga standings at this point.
Mainz's only win of the season came in late November, when they beat Freiburg 3-1 away from home. Since then, Mainz have gone five league games without a win.
They also lost a penalty shootout against VfL Bochum in the second round of the DFB Pokal after that game was tied 2-2 in normal time.
Bayern Munich vs Mainz Head-to-Head
In the last 22 games between Bayern and Mainz, the Bavarians have won 16 and lost four.
With the form that Mainz are in at the moment, Bayern will be confident of extending their dominance in this fixture.
Bayern Munich form guide: W-W-D-W-D
Mainz form guide: L-L-D-L-L
Bayern Munich vs Mainz Team News
Joshua Kimmich and Alphonso Davies gave Bayern a boost by returning to action before the break. Leon Goretzka is expected to be fit as well.
Javi Martinez, Joshua Zirkzee and Tanguy Nianzou are injured and ruled out of this game.
Injured: Javi Martinez, Joshua Zirkzee, Tanguy Nianzou
Mainz have no known injury concerns at the moment, with midfielder Levin Oztunali set to be cleared to face Bayern on Sunday.
Bayern Munich vs Mainz Predicted XI
Bayern Munich Predicted XI (4-2-3-1): Manuel Neuer, Benjamin Pavard, Jerome Boateng, David Alaba, Alphonso Davies; Joshua Kimmich, Leon Goretza; Kingsley Coman, Thomas Muller, Serge Gnabry; Robert Lewandowski
Mainz Predicted XI (5-3-2): Robin Zentner; Edimilson Fernandes, Jerry St. Juste, Alexander Hack, Moussa Niakhate, Daniel Brosinski; Leandro Barreiro, Pierre Kunde, Jean-Paul Boetius; Robin Quaison, Jean-Philippe Mateta
Bayern Munich vs Mainz Prediction
The only thing that could stop Bayern from completely blowing Mainz away could be the rust from not having played for 10 days.
However, that rest may have been invigorating for the league leaders who have endured a busy campaign that included Champions League football.
We predict a comfortable win for Bayern Munich at home.
Prediction: Bayern Munich 4-0 MainzPublished 02 Jan 2021, 12:08 IST