Manchester United and Barcelona, titans of European football, face off in what could be the tie of this season's Champions League. But who will win?

The last time these two great clubs played one another was in the 1998-99 season, drawing 3-3 twice. Of course, Manchester United went on to become champions of Europe then. In the treble season, the Red Devils were undefeated in Europe, and in the current campaign, they have yet to be beaten. Their table-topping domestic form coupled with eight victories in Europe has seen the premiership champions tagged as favourites for the tie, but are we underestimating Barcelona?

Difference in Form

Barcelona's mesmerizing form of recent seasons has largely been absent this campaign. They are behind Real Madrid by nine points in the league, and, where Manchester United have arguably been the most impressive side in Europe this season, Barcelona's indifferent Champions League form certainly suggests that they may be incapable of overcoming the might of the Red Devils.

This season Manchester United seem to strike fear into their opponents, which is perhaps evident in the sounds emanating from Camp Nou. The Blaugrana fans fear a demolition from the premiership leaders. This has led figures such as the Barcelona striker Thierry Henry and former Barcelona manager Johan Cruyff to publicly defend the club's chances. As reported by Lucas Brown for Goal.com on April 14, Cruyff rebuked, "Are they going to hammer us? Why would anyone think that? Where is the evidence? While I really do think Manchester United are the best of the three English sides left in the competition, I am absolutely convinced that they think that same thing about Barca".

Be that as it may, results speak for themselves. Barcelona have one solitary victory of their previous seven Primera Liga games. This season has seen Manchester United defeat Arsenal twice, including the 4-0 destruction of their London rivals in their February FA Cup meeting. In the league they have beaten Chelsea, as well as defeating Liverpool twice. Whereas the Red Devils swept aside Roma in the quarter finals of the Champions League, Barcelona's vulnerabilities were glaringly exposed as they scraped past Schalke.

Where Manchester United can be Beaten

Although Ronaldinho is ruled out of the semi-final clash through injury, crucially for the Catalan club, the sensational Lionel Messi is expected to play. The Argentinean playmaker was in scintillating form before his recent injury, and he will be Manchester United's main cause for concern. The recent 2-1 victory over Arsenal showed United's ability to win even when playing poorly. However, their performance will encourage Barcelona. The game has shown that they are not infallible. The importance of Vidic to United has become painstakingly clear in his absence and that game has raised questions as to whether the 33 year old Paul Scholes can still produce against the top teams.

In recent seasons the premier league leaders have had a penchant for becoming over-awed against Europe's finest clubs. Last season against AC Milan in the San Siro, Ferguson's men froze, and they were humbled in a 3-0 thrashing. Against Real Madrid in 2003 in the Bernebeu, it was the same result. While Roma are a formidable team it is fair to say that Manchester United have not been tested in Europe this season against a team the standing of Barcelona.

The Two Finest Attacking Sides in Europe

Barcelona have an outstanding attack, boasting Messi, Thierry Henry and Samuel Eto'o. If they can play to their strengths and retain possession they can stifle Manchester United's game. While there is an anticipation of greatness from Old Trafford, the Catalan side are perhaps the only club capable of stopping United from becoming European Champions for a third time. However, if Ferguson's men can soak up Barca's attack, and United's big names perform to their ability, they will create chances. With Ronaldo and Rooney Manchester United have perhaps the most dangerous counter-attacking team in Europe, and this is where they can hurt Barca.

Carles Puyol, the Barcelona skipper, has said of the tie, "This could be the greatest semi-final in the history of the Champions League". It certainly has the potential to be so.