Both sides exceeded expectations to claim top-half finishes last season, and they look set to achieve that once more having each won on the opening weekend last time out.
Match preview
After flirting with relegation for several seasons, Mainz's form has been dramatically improved since appointing Bo Svensson as head coach in January 2021.
The former Denmark international, who played 109 league games for Mainz as a player, brilliantly guided Die Nullfunfer away from the relegation zone during his first few months in charge, before helping them to their highest finish since the 2015-16 season when finishing eighth last campaign.
On the evidence of last weekend's 2-1 win away to VfL Bochum, Mainz fans can focus on securing another top-half finish rather than worrying about being involved at the wrong end of the table. Two brilliant headers from Karim Onisiwo either side of half-time earned his side a hard-fought victory following Kevin Stoger's fortuitous equaliser against his former club.
Saturday's home match against Union Berlin is likely to be a far tougher test, though, with Svensson aware his side's performance levels will need to increase in order to gain another positive result.
Indeed, Union Berlin made it four derby wins in a row against their city rivals Hertha Berlin with a comfortable 3-1 victory on the opening weekend.
The Berlin derby marked the first time since 1965 that a city rivalry occurred on the opening weekend of a Bundesliga campaign, with Urs Fischer's side rising to the occasion early doors.
Theoson Siebatcheu highlighted what an excellent signing he could prove to be by delighting the home fans with a derby goal on debut, with the former Young Boys forward expertly meeting Sheraldo Becker's cross at the near post to head the ball beyond the reach of Oliver Christensen.
Becker doubled the hosts' lead five minutes after half-time by cheekily lifting the ball over the onrushing Christensen, with Janik Haberer proving pivotal once again with his range of passing in the build-up to both goals.
Robin Knoche - who scored the winner against Hertha in the DFB-Pokal last season - was at it again with a bullet header to wrap up the three points, with Dodi Lukebakio's late consolation strike far from dampening any spirits among Die Eisermen's camp.
Having missed out on Champions League football by a single point last campaign, there is clearly real optimism surrounding Fischer's side, but for now they will take things one game at a time.
Mainz 05 Bundesliga form:
Mainz 05 form (all competitions):
Union Berlin Bundesliga form:
Union Berlin form (all competitions):
Team News
Mainz should be boosted by the return of club captain Silvan Widmer for the visit of Union Berlin, with the wing-back potentially replacing Edmilson Fernandes from last weekend's starting XI.
Otherwise, Svensson is unlikely to change a winning formula, especially with Jonathan Burkardt - who scored 11 league goals for the club last season - still unavailable for at least one more week due to a muscle strain.
Likewise, Fischer will almost certainly name an unchanged side for his side's trip to Mainz, with Timo Baumgartl and Fabio Schneider unlikely to be able to be a part of the squad.
Siebatcheu and Becker's growing strike partnership will cause many opposing defenders to have sleepless nights, with their combination of pace, power and finishing ability a potential recipe for success for Union Berlin this season.
Mainz 05 possible starting lineup:
Zentner; Bell, Hack, Leitsch; Widmer, Kohr, Stach, Aaron; Lee, Fulgini; Onisiwo
Union Berlin possible starting lineup:
Ronnow; Jaeckel, Knoche, Leite; Trimmel, Haraguchi, Khedira, Haberer, Ryerson; Becker, Siebatcheu
We say: Mainz 05 1-1 Union Berlin
Both sides have considerably punched above their weight in recent times, and we can envisage a share of the spoils on Sunday.
It could be an interesting tactical battle with both managers lining their sides up in similar formations and preferring to counter rather than control, with the visitors perhaps having the edge in attack.