Algeria were surprise advancers from Group H behind Belgium and will now face Germany for a place in the quarter-finals.
"If anybody thinks or believes - and I think this is a feeling among the general public - that Algeria are easy opponents and that we can already start thinking of the next round, then they're making a huge mistake," said Low.
"We must remain focused and on our toes because any complacency will be punished.
"I took a look at Algeria yesterday for an hour and I know we're up against a very compact side who run a lot and are aggressive."
Germany finished top of Group G with seven points. The winner of Monday's match will face either France or Nigeria in the last eight.