Lazio took an early lead at Estadio Das Antas through Claudio Lopez, but the Italians were simply overwhelmed as Porto came storming back with two goals in either half.
The Italians can look to a second leg in the Eternal City, but they face a mountain to climb if they are to reach the final.
Lazio opened the scoring when their gifted forward line broke beyond Porto`s defence on six minutes and Claudio Lopez lashed home a low shot from Giuseppe Favalli`s cross.
The setback sparked Porto into life and Maniche levelled the scores on ten minutes when he curled a 25-yard drive past Angelo Peruzzi.
Lazio seemed stunned by the setback, as they retreated further back and allowed Porto to play.
Derlei had a goal disallowed for offside and Peruzzi had to rush off his line to clear as Deco raced in on goal.
Peruzzi`s positioning was questioned for the first goal, but he atoned for the error with a stunning reflex save to thwart Deco.
However, from the resulting corner, Porto took the lead as an unmarked Derlei met Deco`s set-piece and headed downwards and his effort deflected off Jorge Costa before flying into the net.
Porto should have been out of sight by the interval, but the hosts wasted a number of gilt-edged chances.
Helder Postiga (pictured) shot wide, Jorge Costa headed over, Derlei went close and Peruzzi pulled off an amazing one-handed save to thwart Postiga.
The inclement weather continued after the break, as did Porto`s flowing football and they eventually added to their lead on 49 minutes.
Eyebrows were raised when Deco fired in a free-kick from all of 40 yards, but Peruzzi spilled the ball as it landed in front of him and Derlei gleefully rammed home the rebound.
Lazio were by now a forlorn team and Postiga punished them further by smashing home a fierce right-foot shot after ghosting through the defence.
Porto will surely fancy themselves to reach the final, particularly if they travel to Italy in the same form.
Click on fixture for full match summary
Porto v Lazio
