“We’re still within striking distance (of Bayern) and want to keep attacking,” said Leipzig coach Nagelsmann.
Mainz are second-from-bottom and winless in their last nine games but gave Bayern a huge scare three weeks ago, taking a two-goal lead before losing 5-2 at Munich’s Allianz Arena.
However, Nagelsmann says his team were “incredibly mature” on Wednesday against the stubborn defence of Union Berlin, which eventually cracked in Leipzig’s 1-0 home win.
Ten minutes into his cameo appearance on Wednesday, Emil Forsberg combined with Dani Olmo to unpick the Union defence with great footwork.
Nagelsmann dubbed the pair “phonebox kickers” for the close-quarters skills which led to Forsberg’s winning goal, and the Swede could feature at the weekend.
Bayern meanwhile are at bottom side Schalke on Sunday with goalkeeper Manuel Neuer chasing a league record at his former club.
On Wednesday, Neuer equalled Oliver Kahn’s record of 196 Bundesliga clean sheets.
But Bayern have looked vulnerable in recent weeks.
They have won their last two league games, but were lucky to get a 1-0 victory at Augsburg on Wednesday after Alfred Finnbogason hit the post with a late penalty.
“We had to fight tooth and nail at the back,” admitted Neuer after his first clean sheet in the league since October.
Coach Hansi Flick admitted that “it’s been a long time” since his side has dominated matches as much as last season, when Bayern won the treble of Champions League, Bundesliga and German Cup titles.
At the halfway stage of the campaign, Schalke are level on seven points with Mainz, 10 away from the guaranteed safety of 15th place.
American striker Matthew Hoppe, 19, will try to test Neuer after grabbing headlines by scoring five goals in his last three games.
The Royal Blues have beefed up their attack by bringing back Klaas-Jan Huntelaar.
The Dutch veteran, 37, who netted 126 goals for Schalke from 2010 until joining Ajax in 2017, hopes to shake off a calf strain to face Bayern on Sunday.
One to watch: Moussa Diaby
Leverkusen’s French winger scored arguably the goal of the midweek round of matches in Tuesday’s 2-1 win over Dortmund with a superb finish after Leon Bailey’s diagonal pass.
Great work by Diaby, 21, then put Florian Wirtz into space to score the winning goal to end Leverkusen’s winless run of four league games.
22: Goals Bayern striker Robert Lewandowski has scored in 17 league games this season. At the same point of the 1972/72 season, in which Gerd Mueller set the all-time record of 40 goals, ‘The Bomber’ had scored 17 times.
422: Matches Bayern goalkeeper Manuel Neuer needed to equal the league record of 196 clean sheets, set by ex-Germany keeper Oliver Kahn, who took 557 games.
4 – At 17 years, 261 days old, Leverkusen midfielder Wirtz, who is still at school, is the youngest player to score four Bundesliga goals, beating the previous records of Kai Havertz and Timo Werner, who are now both at Chelsea.
Fixtures (1430 GMT unless stated)
Borussia Moenchengladbach v Borussia Dortmund (1930)
Bayer Leverkusen v Wolfsburg, Freiburg v Stuttgart, Mainz v RB Leipzig, Augsburg v Union Berlin, Arminia Bielefeld v Eintracht Frankfurt, Hertha Berlin v Werder Bremen (1730)
Schalke v Bayern Munich, Hoffenheim v Cologne (1700)