Chief executive Hans-Joachim Watzke has cautioned Borussia Dortmund against the prospect of a Porto backlash in the second leg of their Europa League tie on Thursday.
Dortmund hold a commanding 2-0 advantage in the last-32 contest having overcome Porto at Signal Iduna Park last week.
But Watzke believes Thomas Tuchel's men will come unstuck if they are not fully focused on the task in hand at Estadio do Dragao.
"We have to be careful," he said. "I have seen Porto in home fixtures three times in the last few years.
"They can develop a lot of pressure, are very strong in their stadium and they manage to spur their fans.
"So we have to be very alert. If we are very alert we should make it but if we think we could do with just 95 per cent [effort] it will become tough."
Dortmund defender Marcel Schmelzer, who completed 90 minutes in the first leg before sitting out Sunday's win at Bayer Leverkusen among the substitutes, agrees with Watzke's assessment.
"We didn't win this tie yet," he said. "We are very keen to get as far as possible but now we are facing Porto, a very strong side, which we will try to eliminate tomorrow.
"We know that Porto is a great team. In their home match they will build up a lot of pressure and will try everything to turn the fixture around.
"So we have to be very alert to decide to fixture in our favour."