Mats Hummels has lauded his former team-mate Robert Lewandowski ahead of Borussia Dortmund's table-topping clash with Bayern Munich next weekend.

The Poland international followed up his record-breaking five-goal haul against Wolfsburg in midweek with a double against Mainz, taking his Bundesliga tally to 101 goals in 168 appearances.

Lewandowski netted 74 of those while playing alongside Hummels at Signal Iduna Park, winning back-to-back Bundesliga titles under Jurgen Klopp before making the switch to Munich.

And the World Cup-winning centre-back admitted he can still enjoy the displays of his former team-mate, insisting there are no hard feelings despite his switch to their rivals.

"I had an incredulous smile on my face when Lewy [Lewandowski] scored his five goals, since I don't see the shirt or the club for which he plays, just an exceptional striker," Hummels told Bild am Sonntag.

"You can enjoy his goals, even as part of BVB. He is a complete machine.

"No matter what he does, he doesn't have to answer to anyone. He did something unpredictable and extraordinary at BVB."

After a dismal 2014-15 campaign which culminated in Klopp leaving the club, Thomas Tuchel took the helm at Dortmund and has quickly led them back towards the upper reaches of the Bundesliga table.

They take on leaders Bayern in their biggest game of the season so far next Sunday, and, while Hummels insists Dortmund's aim is not to challenge for the title, he is adamant they have been restored to their confident best.

"I do not feel that we are wasting our talent and our quality. With the team that we have, we need to at least take third place - probably even better. Our goal must be to be merciless," he added.

"Our confidence is back. We are very aware of our qualities again."