Bayern shift through the gears with Davies. He starts the move and ends it - just not how he would have liked. He stumbles into the ball rather than striking it, and Rönnow gets down to block.
Union haven't created much, but they're so close to taking the lead. Leweling outpaces and outmuscles Upamecano on his way through on the Bayern goal. He tries to catch out Neuer at his near post, but Bayern's skipper throws out a steel-like wrist to push it behind.
Union have made life very difficult for Coman. Can Gnabry find a way through?
Gravenberch catches Ryerson late. It's the toe rather than shin, though. Had it been the latter, the Bayern sub might have been shown his marching orders.
DOUBLE CHANGE - UNION
Becker begrudgingly makes way, with Leweling on alongside Michel in the Union attack. Behrens also heads off.
DOUBLE CHANGE - BAYERN
Here comes Müller for his 633rd competitive appearance as a Bayern player. He's now outright second among the record appearance-makers. Gravenberch is also on, after impressing on his full debut in the cup in midweek. Musiala and Kimmich make way.
MÜLLER PRIMED AND READY
Thomas Müller is about to come on for Bayern...
BAYERN TURNING THE SCREW
Union have largely kept Mane quiet, but he fashions an opening before pulling a shot wide.
HALF-TIME! UNION 1-1 BAYERN
It's all square in the capital after Kimmich cancelled out Becker's early goal. Union have given little away, and will be quietly confident they can get at least a draw. Nagelsmann may already be thinking about making changes.
Bayern attack up the right-hand side with Pavard. Musiala collects and feeds Sane, who goes for precision over power and curls wide of the far post.
Becker controls a miscued Davies clearance and crosses towards the corner of the six-yard box. Behrens helps it on to Ryerson, but Pavard gets there first. Crucial intervention.
ROUGH AND TUMBLE
Knoche's lucky to avoid a booking after going through the back of Mane. That's one way to stop the Senegal star. Bayern were in pass-and-move mode for a moment.
UNION HOLDING FIRM
Musiala skips the challenges before sliding a ball into the box for Sane. He catches it really well, but Rönnow diverts the Bayern No.10's low strike to safety with his trailing foot.
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Union aim long for resident speedster Becker, but de Ligt tracks his run and pokes the ball in to touch.
Kimmich is the first player into the referee's black book. The Bayern terrier caught Thorsby a touch late and high.
TWO DIFFICULT CHANCES TAKEN
Don't poke the bear! Musiala's shot following a corner is blocked back out to set-piece taker Kimmich. It's a difficult angle, but he makes it look easy with a low drive that whistles through the ruck of bodies and into the bottom corner. Game on!
That's five Bundesliga goals for Becker already this season - a league high and personal best. Fischer might be wondering why he didn't start playing him as a striker sooner!
BECKER STRIKES FIRST!
Union lead the record champions! Trimmel floats a free-kick from left to right. Becker is unmarked and sends a fablous side-footed volley into the far corner. Neuer barely moved!
Never one to shy away from a shot from distance, Pavard win's Bayern's first corner with a deflected drive. Kimmich's first effort towards the near post is put behind for another. De Ligt attacks the second delivery, but Union smuggle it clear.
Becker with the chance to run at the Bayern defence for the first time. He's surrounded by Bayern shirts, but still manages to dig out a cross that de Ligt shepherds behind. De Ligt is first to the ensuing delivery. Becker then lumps a return pass into the arms of Neuer.
STRONG BAYERN START
Davies advances and fires in a low cross. Musiala can't bring it under control, though, and Khedira hacks clear. Sane then has a shot charged down by Knoche, before Sabitzer shoots wide from range.
The ball's rolling in Berlin. Union are in red and white stripes, Bayern in navy blue with red flashes.
The players gather around the centre circle in an act of remembrance, 50 years on from the terrorist attack on the Israeli Olympic team at the 1972 Munich games.
HERE COME THE TEAMS!
Superb atmosphere as the teams take to the field in Köpenick. The winner - if there is one - will go top of the Bundesliga!
The 22,000 home fans belt the out club's anthem, 'Eisern Union', or 'Iron Union'. Remarkable to think this is still only their fourth Bundesliga season. Feels like they've been around a lot longer.
WATCH: ALL JAMAL MUSIALA'S BUNDESLIGA GOALS
FINELY TUNED MACHINE
Bayern have averaged 61 percent possession in their Bundesliga matches so far this season and have completed 88.7 percent of their passes from open play. Little wonder they have only allowed opposing teams a league-low 34 shots at goal this term.
Union are the only Bundesliga team not to fall behind in a game this season. What's more, they've won a league-best 120 aerial duels, covered more ground than anyone else (72.4 miles per game) and also committed more fouls than any other team (59).
UNION TEAM NEWS
The hosts have made four changes to the team that hammered Schalke last weekend. Behrens, Schaefer, Trimmel and Jaeckel come in for Jordan, Haberer, Gießelmann and Leite. USMNT international, Jordan, is sidelined with a muscular issue.
BAYERN TEAM NEWS
Bayern boss Julian Nagelsmann has almost a full squad at his disposal today and Union will have to deal with an attacking quartet that will surely have the visitors sweating this afternoon. Coman, Musiala and Sane will look to interchange with Mane as the record champions look to add to their 16-goal haul.
Grill, Gießelmann, Puchacz, Öztunali, Seguin, Haraguchi, Michel, Pantovic, Lewelling
Rönnow - Doekhi, Knoche, Jaeckel - Trimmel (c), Thorsby, Khedira, Schäfer, Ryerson - Becker, Behrens
Ulreich, Gnabry, Goretzka, Choupo-Moting, Hernandez, Müller, Gravenberch, Mazraoui, Stanisic
Neuer - Pavard, Upamecano, de Ligt, Davies - Kimmich, Sabitzer - Coman, Musiala, Sane - Mane
WATCH: JORDAN - A STAR IN STRIPES
WATCH: BAYERN 4-0 UNION
WATCH: UNION 2-5 BAYERN
WATCH: BECKER OOZING CONFIDENCE
FEET ON THE GROUND
Despite finishing fifth last season and having already accounted for Leipzig in 2022/23, top-four murmurs are being brushed aside in the Union camp. "Our first goal is [remaining in] the Bundesliga," said top scorer Sheraldo Becker. "We want to show everybody we are a team, we are stable [and that] we're here to stay."
Fischer is without defender Diogo Leite, so Paul Jaeckel should come in in the only change to the Schalke romp. Julian Nagelsmann made a number changes in midweek, and is expected to recall the likes of Manuel Neuer, Benjamin Pavard, Alphonso Davies and Marcel Sabitzer. Mathys Tel, who became Bayern's youngest ever goalscorer, aged 17 years and 126 days old, misses out with a knock.
WATCH: SADIO MANE - WORLD STAR
"Bayern have a lot of players that are extremely quick, players that can decide a game. It will take courage, both with and without the ball. But we're not thinking about a top-of-the-table clash, we just want to play as well as we can."
"Union have developed well, playing direct football to their two strikers. It's a great stadium with amazing fans. We're going to try our best to compete. It's challenging to play there, but doable. And we will make changes after the cup game."
Bayern are unbeaten in their six Bundesliga meetings with Union (W4, D2). They are the only team to have recorded two Bundesliga away wins at the fortress Alten Försterei.
WATCH: NINETEEN SAVES OF SOMMER
WATCH: BAYERN 1-1 GLADBACH
WATCH: SCHALKE 1-6 UNION
THE SEASON SO FAR
Union have made a flying start to 2022/23, beating Hertha (3-1), Leipzig (2-1) and Schalke (6-1), whilst drawing 0-0 with Mainz. Bayern were held to a 1-1 draw by Gladbach last time out, after posting wins over Frankfurt (6-1), Wolfsburg (2-0) and Bochum (7-0).
These are heady days for Union Berlin! Urs Fischer's side are level on 10 points with Bayern Munich going into Matchday 5, and will go top with a win. Stay tuned!