The Heineken Cup - Europe's leading club rugby competition -  enters the semi-final stage this weekend as Saracens, Clermont, Toulon and Munster battle it out for a place in the northern hemisphere's premier final.

First up this weekend, Saracens take on last year's losing finalists Clermont Auvergne at Twickenham in what should be a thrilling encounter. Both teams arrive off the back of only one loss throughout the tournament to date and despite what is effectively home advantage for Saracens, there is little to chose from between the two teams.

Both teams arrive in the semi-finals off of tight wins, Saracens beating Ulster 15-17 at Ravenhill while Clermont survived a late Leicester fightback to win 22-16 and extend their incredible unbeaten run at the Stade Marcel-Michelin to 76 matches.

Both sides will also want to erase the disappointment of last season's Heineken Cup campaign when Sarries were downed by Toulon in the semi-finals and Clermont were outdone by the same team in the final showpiece.

In their respective domestic leagues, Saracens sit top of the Premiership nine points clear while Clermont are in third, five points adrift od leaders Toulon with all to play for. Come Saturday this wont count for much, as over the 80 minutes whoever turns up will advance. There are no second chances.

Saracens are a cracking outfit and the decision making and boot of England fly-half Owen Farrell will be key to their aspirations. Under pressure there is no question he maintains the accuracy required to convert opposition mistakes into points, however in open play at times his decision whether to run or kick has been questionable. On Saturday, any mistakes or doubts of this nature will be seized upon by Clermont, a team that will punish the opposition under these circumstances.

Saracens have a powerful mobile pack that could dominate both the scrum and the line out. In Schalk Brits they have an extra loose forward that is full of running and a nuisance at the breakdown.

Billy Vunipola is the man that Clermont will want to keep an eye on. The Australian-born England international has been in sparkling form this season both domestically and internationally and another powerful performance by the number eight could prove the difference between the two sides.

His ability to carry the ball and take out defenders opens up plenty of space for the outside backs and without question this attribute has played some part in Chris Ashton leading the try scoring charts in the Heineken Cup with nine to date.

Clermont were beaten by a point in last season's final which will be a serious motivational threat to Saracens. The fact that they haven't been beaten at home for 76 matches suggests that this side is one that has the credentials required to lift the coveted trophy.

The experienced half-back pairing of Morgan Parra and Brock James will pull the strings for the French and you can bet James will be looking to distribute the ball as often as possible to the brilliant Wesley Fofana. Few would argue Fofana is the best outside centre in world rugby and his ability to time his runs and glide through the opposition defence is their main threat.

Lurking in support outside Fofana will be former All Black wing Sitiveni Sivivatu, one of the most potent attacking forces in Europe.

The Fijian-born flyer is one of Europe's leaders in meters gained and offloads this season in the Heineken Cup and has a canny ability of appearing where you least expect him - something that will create a few headaches for the Saracens defence.

Come what may at Twickenham on Saturday, it will be a tight battle, perhaps decided by a missed kick, a lack of discipline or sheer individual match-winning brilliance. Whatever the outcome, expect a high octane and physical encounter of the highest order.

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