Sir Alex Ferguson admits apprehension ahead of Manchester United's Champions League semi-final against underdog German team Schalke 04 Reuters

Sir Alex Ferguson has warned his Manchester United side there could be serious repercussions if they allow themselves to underestimate Schalke 04 in the first leg of their Champions League semi‑final on Tuesday.

Ferguson has galling memories of playing German clubs at this stage of Europe's premier club competition, losing to Borussia Dortmund in 1997 and Bayer Leverkusen in 2002, and the United manager, describing himself as "apprehensive", has instructed his players to guard against thinking they have been fortunate to avoid Real Madrid and Barcelona in the last four and are certainties for the final at Wembley next month.

Schalke have not won the Bundesliga title since 1958 and are in 10th position, having spent parts of the season hovering close to the relegation zone, but their domestic troubles have been eclipsed by their form in the Champions League, beating the holders Internazionale 7-3 on aggregate in the quarter-finals.

"They [Schalke] have beaten every team they have played in the Champions League at home this season, including Valencia and Inter Milan," Ferguson said.

"Taken in isolation, that's fantastic form and there is no way Manchester United are going to be led into believing that this is easy. This is a difficult game for us and we have no choice but to treat it that way."

Ferguson has shown his players the video of Schalke's remarkable 5-2 win against Inter at San Siro as well as one of their second-leg 2-1 victory at the Veltins- Arena. The United manager had travelled to Germany for that match and he spoke of Schalke playing with a "self‑determination" that makes them the surprise package of this season's competition.

"They never looked like losing the game and that's quite a credit to them in terms of playing a side like Inter Milan," the Scot said.

"They always looked in complete control and, even in the good spells that Inter had, they [Schalke] never looked like losing. It's a semi-final now and, any semi‑final you go into, you know it is going to be difficult."

Ferguson, nonetheless, can be encouraged by the momentum that has taken United to the verge of a record 19th league title, six points clear of second-placed Chelsea with only four games remaining.

Dimitar Berbatov, nursing a groin strain, was the only unexpected absentee on their flight to Germany and, though Ferguson talked of being disappointed with United's away record in the Premier League, they have won 10 of their past 12 excursions in Europe.

They also have what he believes could be a considerable advantage of the second leg being at Old Trafford, where they have not lost for over a year. "We are disappointed with a lot of our games away from home but no one can point a finger at our home form because it has been absolutely sensational," Ferguson said.

"If you talk about our away form then, yes, I can see it some of the time but you can't criticise our home form. And what you are seeing, as you did again on Saturday [in the 1-0 Premier League victory against Everton], is that this team will not give in. There's absolutely no chance this team will ever give in and that's a great quality."

United's fourth semi-final in five seasons will take Rooney back to the stadium where he was sent off for England in the 2006 World Cup quarter-final against Portugal. Ferguson claimed he was not aware of that fact and also made the surprising admission that he had never seen the incident. The United manager was asked whether it would be on Rooney's mind and shook his head. "You have to put bad moments like that behind you," he said. "That's normal for any footballer."

Rooney's form, he said, had been "absolutely fantastic for the last two months" and it speaks volumes about the way Berbatov has been marginalised that the absence of the Premier League's leading scorer did not merit a single mention.

"Wayne, more than anyone, realises that performances are the thing that he will always be judged on because there is an expectation of the boy," Ferguson said. "You can see that he has stepped up to the mark and proved himself entirely.

"But the most important thing has been the freshness that's come into the team with players like Antonio Valencia, Rio Ferdinand, Anderson and Park Ji-sung coming back [from injury]. It lifts you when the big competitions come along and all your best players are fit. It can cause a selection problem but we are glad to have them."