Bayern Munich has been almost untouchable in the German Bundesliga but its edgy performance against Arsenal suggested that the lack of competition at domestic level could hinder its European ambitions.

Bayern again found the Champions League a different proposition as it struggled to break down a surprisingly cautious Arsenal in the first half of Tuesday's game and showed vulnerabilities in defense when the visitors came at them in the final half hour.

The last time Bayern failed to win a competitive game was also in the Champions League last December, when it squandered a 0-2 lead and lost 2-3 at home to Manchester City.

Arsenal manager Arsene Wenger also faced Bayern last season in the round of 16 and was able to make a brief comparison between last year's Champions League-winning side, coached by Jupp Heynckes, and Pep Guardiola's version this year.

"Bayern are a good side but they were more vulnerable this season than last year," he told a news conference after the game.

"I thought Bayern were very vulnerable defensively, we didn't take advantage of that. I felt the situations were there where we could have made more of it."

Bayern, leading 2-0 from the first leg, went ahead 10 minutes into the second half with a Bastian Schweinsteiger goal but Lukas Podolski stunned the Allianz Arena when he levelled two minutes later.

Guardiola's team immediately lost its swagger as the confidence drained out of them and Arsenal threatened a second.

The weaknesses, however, have not been apparent in the Bundesliga where Bayern's statistics reflect an almost embarrassing dominance.

Last season's treble winner has won 22 and drawn two of 24 league games this season and scored 72 goals, including 15 in its last three games alone. Its last defeat in the league was against Bayer Leverkusen in October 2012.

Meanwhile, when Atletico Madrid was knocked out in the Champions League quarter-finals by Ajax Amsterdam in 1997, midfielder Diego Simeone could scarcely have imagined it would be 17 years before it reached the last eight again.

Almost two decades later, Simeone is now coaching the La Liga side and Tuesday's dominant 4-1 home victory against seven-time winner AC Milan, which put Atletico through to the quarterfinals 5-1 on aggregate, was the latest evidence he has moulded his unfancied squad into genuine European contenders.

In much the same way as he did as a combative midfielder for Argentina and clubs including Atletico, Inter Milan and Lazio, Simeone's players harried and hassled Milan from the start and they looked dangerous nearly every time they broke forward.