In the battle for the four semi-final spots in the UEFA Champions League, all eyes will be on Tuesday's matches in England, where the ties at Chelsea and Liverpool are still wide open. Wednesday's fixtures would seem to be more clear cut, with Manchester United and Barcelona very much in pole position.

For the third time in this year's competition, an English club will be hoping to prosper against Italian opposition, with Manchester United attempting to follow in the footsteps of Liverpool, who beat Inter Milan (2-0, 1-0) and Arsenal, who eliminated AC Milan (0-0, 2-0) in the Round of 16, without conceding a single goal.

The real showdown will of course be the third match in the space of a week between Liverpool and Arsenal, with the first two ties having ended 1-1. This time, one of the two perennial rivals will have to emerge victorious.

The big match
Liverpool- Arsenal (1-1), Anfield, 20:45 CET

Twice in four days, and in two different competitions, Arsenal failed at home to overcome a perfectly organised Liverpool side who seem to be peaking at just the right time. Rafael Benitez even allowed himself the luxury of rotating his squad and making eight changes (three more than Arsenal) from the first game to the second, and giving such stars as Steven Gerrard and Fernando Torres a well earned breather.

The Reds fell at the final hurdle of this competition last year and are out of the running for this season's Premier League - two reasons why they will be all the more determined to repeat their UEFA Champions League success of 2005. They have a precious away goal in the bank, meaning that Arsene Wenger's men will have to take more risks than perhaps they would like.

Other matches
Chelsea are another team rounding into form at the business end of the season. The Blues climbed to within three points of Manchester United in the Premier League with a 2-0 win at Manchester City on Saturday. After a dream first half last Wednesday, the Londoners were caught out twice by a Fenerbahce team buoyed by its home crowd. The Istanbul outfit are a different proposition away from home, though, (with two defeats and two draws in this year's competition), and their last win in England dates back to October 1996. For all of that Chelsea will be on the guard on Tuesday; after all they are facing a team coached by the legendary Zico.

AS Roma, for whom Francesco Totti is still injured, will be travelling to Old Trafford in the hope of erasing the painful memories of last year's 7-1 thrashing. Having lost the home leg 2-0, Roma have nothing to lose and will be hoping for an early goal to unsettle a Mancunian defence which, deprived of the injured Nemanja Vidic, looked less than convincing in the 2-2 draw at lowly Middlesbrough on Sunday. Sir Alex Ferguson may also be without his other central-defensive lynchpin, Rio Ferdinand. The England captain limped off during the second half at Teesside and may well be replaced by the young Spaniard Gerard Pique.

Barcelona, who have been struggling at the Camp Nou all season, as illustrated by Sunday's draw against Getafe, merely have to defend the lead which they brought back from Gelsenkirchen last week. The "Ronaldinho affair" finally seems to have been sorted out: the player is out injured until the end of the season, which removes, at least temporarily, the cloud of uncertainty regarding his future. Lionel Messi's imminent return from injury will come as good news to the frustrated Blaugrana fans, but his team-mates will have to make sure there are no slip-ups against Schalke if the Argentinian boy wonder is to have a chance to shine in the remainder of this year's Champions League.

Player to watch
17-year-old Bojan Krkic became the second youngest goalscorer in the history of the tournament when he scored for Barcelona against Schalke last week - a feat that saw him immediately elevated to the status of "fifth musketeer" up front for the Catalans. He already has eight goals to his name in La Liga but it will be interesting to see how he handles the increased pressure at this stage of the season. "He already looks like a star even though he is still very young," said coach Frank Rijkaard. "He just needs to keep his feet on the ground and carry on working hard."

The words
"In the first leg, getting an equaliser straight away was the main thing. Scoring an away goal in the Champions League is always very important. The tie is still in the balance but we find ourselves in a good position. And playing at Anfield is always a real advantage." ( Rafael Benitez, Liverpool coach).

The numbers game
Chelsea are unbeaten home in European competition for the past 26 months, with a record of seven wins and three draws in ten Champions League matches.