Chelsea remain on course for the last 16 of the Champions League as an own goal has given them a 1-0 half-time advantage over Porto at Stamford Bridge.
The home side survived an early scare when Yacine Brahimi burst into Chelsea territory and hammered an effort towards goal that was pushed clear by Thibaut Courtois.
However, Chelsea were ahead after just 12 minutes when Eden Hazard played in Diego Costa, with the Spanish striker's effort rebounding off Ivan Marcano and into the back of the net after Iker Casillas had made the initial save.
Oscar then felt he should have won a penalty when he appeared to go down under the challenge of Mercano, but referee Cuneyt Cakir waved away his appeals.
Costa's next taste of the action came when he clashed with Casillas midway through the half, earning a yellow card for a trip on the Porto goalkeeper.
Willian teed up international teammate Oscar, who curled an effort narrowly wide of goal on the half-hour mark, as Chelsea kept Porto at arm's length to lead at the break.