Robert Lewandowski admits Bayern Munich "were not good enough" in the second half of their 4-1 win over Arminia Bielefeld as Hansi Flick criticised his side's "careless" nature.

Thomas Muller and Lewandowski scored two goals each at Bielefelder Alm to put Bayern four goals in front by the 51st minute in their first game back after a two-week break.

However, the champions' wait for a Bundesliga clean sheet stretched into a third game after Ritsu Doan pulled one back in a second half that saw Corentin Tolisso sent off late on.

Bayern had chances to further extend their lead but failed to take them and Lewandowski concedes his side took their foot off the gas against the recently promoted side.

"The first 45 minutes we clearly showed that we wanted to win," Lewandowski, who has now scored against every current Bundesliga side, told Sky Sport Germany.

"We scored three goals and made a few good chances. However, Arminia played well with the ball.

"They're showing this season that they're up for a fight and can put together some good performances. 

"We weren't good enough in the second half. We had two or three chances to score, but we made two or three errors. After a long break the first game back is always important."

Muller assisted Lewandowski's second goal and the prolific striker played a part in both of his team-mate's strikes, taking them to a combined 12 goals in 13 games this season.

But while pleased with the link-up play between his in-form attacking duo, head coach Hansi Flick agrees Bayern could have offered a lot more in the second half.

"The first half was close to what we want. But we had about five minutes after where we were too careless," he said at his post-match news conference. 

"I'm not pleased about that and that made our lives somewhat difficult. For me, Thomas Muller with Robert Lewandowski were our two best players in the match. 

"Of course we can defend better than we did, but we're also missing some training sessions. It will be a challenge, but there is no other option and for that reason we'll take it."

Victory for Bayern - their second in a row in the league since suffering a shock 4-1 loss to Hoffenheim in their last away match - moves them to within a point of leaders RB Leipzig.

Bielefeld dropped to 14th, meanwhile, and head coach Uwe Neuhaus is not using the calibre of opposition as an excuse for his side's below-par first-half display.

"The fact that Bayern have another level of quality, that we know," he said. "We knew before the game too that we could lose by a lot of goals. 

"But nonetheless we aren't happy with the result, particularly with the first half. We still have the capacity to improve at the back and in attack."