By Chris Harris

Arsène Wenger believes that Arsenal's half of the Champions League draw is still wide open.

Barcelona and Chelsea look to be on collision course in the Semi-Finals after emphatic victories in the first legs of their Quarter-Final ties against Bayern Munich and Liverpool respectively. But it's a very different story for Arsenal, who drew in Villarreal, and Manchester United, who were held by Porto.

Arsenal are favourites to advance after scoring a potentially priceless away goal at El Madrigal but Wenger is taking nothing for granted ahead of next week's second leg at Emirates Stadium.

"I don’t know [about the Semi-Finals]. I believe that these two ties, Arsenal Villarreal and Man United Porto, are still undecided," he said.

"On the other side it’s a bit different. Barcelona look to be through completely and Chelsea are in a strong position. But that one still isn’t decided because Liverpool can respond and be strong away from home."

Man United's surprise 2-2 draw with Porto at Old Trafford has dented their hopes of completing a clean sweep of trophies this season but Wenger believes the holders can prevail in Portugal.

"It's not an impossible task," he said.

"Man United have enough quality to go to Porto and win. Porto is a difficult place to go, we had them in the groups, and they have the culture of being in the Champions League regularly. They know how to deal with it. But it’s not impossible for Man United."

Copyright 2014 The Arsenal Football Club plc. Permission to use quotations from this article is granted subject to appropriate credit being given to www.arsenal.com as the source 9 Apr 2009