Michu and summer signing Alejandro Pozuelo also registered on the scoresheet, but Gheorge Grozav netted the goal of the game with a late overhead kick for the visitors
The Premier League outfit all-but sealed their place in the group stages of the competition with an excellent performance at The Liberty Stadium.
Wayne Routledge scored twice in the first 25 minutes for the home side and his strikes were sandwiched by a Michu effort, putting the result beyond doubt in the early stages.
And Wilfried Bony and Alejandro Pozuelo rubbed further salt into Petrolul’s wounds in the second half as the Welsh side completed a one-sided victory in style, before Gheorghe Grozav grabbed a stunning consolation in the closing stages.
Swansea manager Michael Laudrup made three changes from the side that lost 4-1 to Manchester United on Saturday, including Bony, Pablo Hernandez and Neil Taylor in his starting XI.
They replaced trio Nathan Dyer, Ben Davies and Jose Canas while Petrolul manager Cosmin Contra - who played under Laudrup at Getafe - made five changes, including the recall of star striker Grozav, who has already netted five goals in the qualifying stages of the tournament.
|By Michael Lightfoot
Michael Laudrup's side could not have had it any easier as they stormed to an emphatic victory at the Liberty Stadium.
Wayne Routledge scored two and could have had a hat-trick inside 40 minutes, but Pablo Hernandez was the star man, running the entire match and was very unlucky not to score.
There was time for a late consolation goal from Gheorge Grozav, a brilliant overhead kick, but the visitors simply were not good enough.
Laudrup’s men ventured forward with increasing regularity in the early stages, and eventually got their reward in the 14th minute when Routledge opened his account.
Petrolul were caught napping after Shelvey took a free-kick, and Michu allowed the ball to run to the back post where Routledge was on hand to curl his shot in, via a deflection from Ovidiu Hoban.
Michu then doubled Swansea's lead eight minutes later when he quickly reacted to the rebound after Hernandez had an effort at the far post saved by Petrolul goalkeeper Pecanha.
And the result was sealed just moments later when a superb team move saw Routledge nod in Swansea's third.
Michu started the move with an excellent tackle in midfield before Leon Britton picked up the scraps and delivered a defence-splitting pass to Hernandez down the right.
The Spaniard simply stood the ball up at the back post for Routledge to head in, and he could have completed a hat-trick before the break only to shoot over when put through on goal by Michu.
Bony turned a late effort wide as a completely dominant Swansea went into the break with a 3-0 lead, and their advantage was quickly extended once play resumed.
Ivory Coast international Bony initially hit the bar two minutes into the second half after showing excellent strength and control, and Hernandez curled an effort just wide moments later.
A rare Petrolul attack saw half-time substitute Abel Camara shoot wide from distance and Michu had an effort deflected over before the fourth goal came thanks to Bony.
The 24-year-old benefited from an excellent Michu cut-back and although his shot was saved, the ball eventually rolled in off Pecanha.
And Pozuelo added a fifth with an exquisite chip with 20 minutes remaining.
Grozav scored a late consolation for the losers, with an outrageous overhead kick, but it mattered little as Swansea took all the momentum into the second leg, to be played in Romania next week.