Pedro struck a debut Chelsea goal to inspire the champions to their first win of the season at West Brom as John Terry's wretched campaign continued.
The Spain winger opened the scoring following his reported £21 million move from Barcelona to set up a thrilling 3-2 win for the 10-man visitors.
Diego Costa and Cesar Azpilicueta also netted before Terry was sent off for bringing down Salomon Rondon after the break.
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The skipper retained his place despite being hauled off in Chelsea's 3-0 defeat at Manchester City last week but doubts will grow over his future under Jose Mourinho.
James Morrison missed a first-half penalty but still scored twice as the Baggies gave Chelsea a scare, but they are still looking for their first win of the new Premier League season.
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Thibaut Courtois returned from suspension while Kurt Zouma replaced the axed Gary Cahill, who dropped to the bench.
Saido Berahino was dropped by Baggies boss Tony Pulis, who decided the striker was not in the right frame of mind after Albion rejected a bid from Tottenham.
The England Under-21 forward is understood to have been happy to play but Pulis opted to replace him with £12 million record signing Rondon.
James McClean and Callum McManaman also started and McManaman taunted Chelsea in the hosts' fast start and won a penalty after 14 minutes.
The ex-Wigan man darted into the box and lured Nemanja Matic into a clumsy tackle which sent the winger sprawling and referee Mark Clattenburg pointed to the spot.
Berahino would have been on penalty duties had he played, so instead Morrison stepped up and his drilled effort was turned over by Courtois' trailing leg.
The Baggies were on top, though, with Chelsea's only danger coming from Costa, who fired an early shot wide.
Craig Dawson headed over after 17 minutes but the visitors look the lead as Pedro made an immediate impact.
He had been quiet but burst to life when Eden Hazard found him on the edge of the box and his shot deflected in off Gareth McAuley.
From then, Chelsea took control and went 2-0 up on the half-hour following a counter-attack of ruthless precision.
Terry headed an Albion corner clear to Branislav Ivanovic, who found Willian scampering clear. The midfielder picked out Pedro and his cross-shot was finished by the unmarked Costa.
The champions looked set for a Sunday stroll at The Hawthorns but Morrison made amends for his penalty miss by dragging the hosts back into the game.
McClean launched a deep cross to Rondon, who acrobatically cut the ball back to Morrison to fire in from 15 yards.
But the visitors regained their composure three minutes before the break.
The disappointing Cesc Fabregas lofted a clever ball to Costa who, wrestling with McAuley, worked it to the unmarked Azpilicueta to find the corner from 15 yards.
Any hopes Chelsea had of a comfortable win went out the window, however, when Terry was sent off nine minutes after half-time.
The defender allowed Rondon to escape after Chris Brunt's excellent ball and clipped the striker, who was clean through, leaving Clattenburg little choice.
Five minutes later and the Baggies grabbed a lifeline when Morrison claimed his second with a fine header from Brunt's cross which flew in off the bar.
The champions were on the back foot as Rondon and McManaman went close while Costa had the chance to finish the game but fired over with 14 minutes left.
Hazard dragged wide and Courtois saved Chelsea in stoppage time when he produced a fine point-blank block to deny Rondon an equaliser.