Manuel Neuer expects Bayern Munich's unbeaten start to the Bundesliga season to come under threat at Schalke on Saturday.
The reigning champions travel to Gelsenkirchen to take on Jens Keller's side after recording four wins and a draw in their opening five top-flight matches under new manager Pep Guardiola.
Bayern also got their UEFA Champions League defence off to an impressive start by beating CSKA Moscow 3-0 on Tuesday.
Neuer has started every game this season and is anticipating a tough challenge when he returns to the Veltins-Arena to take on his former club.
"We can't expect it to be a walkover," the Germany international told Bayern's official website.
"I think it will be a challenge to beat Schalke, especially considering they’re on a roll.
"We know Schalke are performing very well at the moment, so we have to do everything we can to win this game.
"If we think it will be a simple kick-about, we’ll be shown how wrong we are."
Neuer has faced Schalke four times in the Bundesliga since leaving for Bayern in 2011, winning on every occasion.