Chelsea go into half time at The Hawthorns with a 3-1 lead, thanks to an impressive display from debutant Pedro.
Both sides went into the match in uninspiring form, but it was the Baggies who made the better start to the game, as both James McClean and Salomon Rondon missed early opportunities to give the hosts the lead.
The hosts were gifted a golden opportunity to open the scoring when Callum McManaman was brought down inside the penalty box by Nemanja Matic. James Morrison stepped up to take the spot kick, but Thibault Courtois made an impressive save.
Chelsea did not wait long to make West Brom pay for the miss, as debutant Pedro exchanged passes with Eden Hazard before placing a finish between the legs of Jonas Olsson and into the corner of the net.
Pedro was involved again for Chelsea soon after, leading a counter-attack before finding Diego Costa, who fired the visitors' second goal past Boaz Myhill.
The Baggies did not collapse though, and on 35 minutes found a goal through Morrison. McClean crossed towards Rondon from the left, who acrobatically found Morrison inside the area for a smart finish past Courtois.
Chelsea looked impressive on the counter-attack, though, and Willian should have scored the side's third after Pedro played the Brazilian in for a simple finish from the right, only for his effort to go harmlessly wide.
The visitors did get their third goal before the break though, as Cesar Azpilicueta picked up a lay-off from Costa before scoring his first Premier League goal, with Myhill rooted to the spot.