Klopp spent seven years in charge of Saturday's opponents before leaving them for Dortmund in 2008, and a win at Signal Iduna Park on Saturday would mark a sixth consecutive victory over his former employers.
The 46-year-old - who also played for Mainz for 12 years between 1989 and 2001 - sung the praises of his old club and Tuchel, who has been in the Coface Arena hotseat since August 2009.
"They are an established Bundesliga team now and I'm delighted for them," Klopp said.
"They are in very good shape, and Thomas could train any club in the world."
Dortmund would assure themselves of an automatic spot in next season's UEFA Champions League with a fourth straight Bundesliga victory this weekend, and they head into the game in high spirits.
Klopp's men destroyed already-crowned champions Bayern Munich 3-0 at the Allianz Arena in their last league outing, and followed that up with a 2-0 win over Wolfsburg in Tuesday's DFB-Pokal semi-final.
After the disappointment of exiting this season's Champions League in the last eight, a strong finish to what, at times, has been a difficult league campaign would set Dortmund up perfectly for their cup final against Bayern on May 17.
Mainz, meanwhile, have clear goals of their own to chase, with Tuchel's team still harbouring hopes of a top-four finish.
They currently lie seventh, four points adrift of fourth-placed Bayer Leverkusen, but they have been inconsistent in recent weeks, losing three of their last five Bundesliga matches.
A spot in the UEFA Europa League looks well within their reach, though, as they are just two points behind Borussia Monchengladbach in sixth with a trip to face Lucien Favre's men still to come next month.
In November's reverse fixture, Robert Lewandowski scored twice in a 3-1 win for Dortmund and Mainz will have to find a better way to cope with the Bundesliga's joint top scorer on Saturday.
Klopp revealed on Thursday that Dortmund keeper Roman Weidenfeller will miss out with a groin injury, with Mitchell Langerak set to deputise, while the coach also confirmed club captain Sebastian Kehl is doubtful with a knock.
For the visitors, long-term absentees Julian Baumgartlinger (knee) and Dani Schahin (groin) have returned to training, but this weekend will come too soon for the pair.
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