The home side dominated the possession early on and soon took the lead through Wayne Routledge.
The winger struck after receiving the ball 14 yards from goal. His shot took a slight deflection off a defender but ended up in the net.
Two minutes later and the game was all but over as Routledge grabbed his second of the night with a header at the far post.
Routledge should have got his hat-trick before the break but, after being put through on goal, he shot over the bar.
The second half followed the same manner as the first as Petrolul Ploiesti struggled to make any impact.
The fourth goal came via a Pecanha own goal. The Ploiesti keeper blocked Michu's shot but saw the ball go in off his back.
Michael Laudrup then replaced Routledge with Alejandro Pozuelo, who made it 5-0 nine minutes after coming on.
Michu was again involved, slipping the ball to Pozuelo who chipped the ball over the keeper.
Ploiesti managed to pull one back in the dying minutes with an over-head kick from Gheorghe Grosav.