The Ivorian striker had a hand in all of his side's goals as Peter Odemwingie's early opener was swiftly canceled out, with the champions proving too powerful for the hosts
Didier Drogba, starting ahead of Fernando Torres, inspired his team to a first-half comeback after Peter Odemwingie opened the scoring, with the Ivorian scoring one, setting up Salomon Kalou and having a say in Frank Lampard's strike before the break.
Ancelotti dropped both Torres and Nicolas Anelka, and gave Drogba, goalscorer in midweek, the chance to lead the line once more, flanked by Kalou and Florent Malouda, while David Luiz came in for compatriot Alex to partner John Terry at the back. For the hosts, the only change was James Morrison's introduction ahead of Simon Cox.
The opening stages were mostly dominated by the visitor, who looked composed if not entirely probing in possession, but Baggies winger Jerome Thomas produced the two brightest moments of the early exchanges by deftly flicking the ball over David Luiz before bursting past John Obi Mikel on a counter attack, only to be denied on both occasions.
But West Brom fashioned a chance just moments later, again on the break, as Chris Brunt marvelously clipped a ball over the top for James Morrison, who sprinted down the right channel before being shepherded away from goal by John Terry. The Scotsman, however, spotted Youssuf Mulumbu in acres of space, but the midfielder could only blast over from 18 yards out.
Chelsea had a chance of its own moments later as Kalou burst through a gap down West Brom's left and latched onto Drogba's reverse ball, but the attacker could only fire over just inside the area.
And the champions were made to rue that wastefulness as the Baggies took the lead minutes later with Odemwingie finishing off a swift counter-attack by picking up Thomas's deflected through ball and expertly lifting over Petr Cech on 17 minutes.
But that lead was to last only minutes as Chelsea equalized through Drogba, who powerfully thumped home after Nicky Shorey's woeful clearance from a Malouda cross.
Top Drog | Excellent Didier stole the show by having hand in all three first-half goals
Drogba continued to show why he should still be the Blues' top dog with a surging run and shot that Scott Carson could only palm into the path of Kalou, who drilled it back across the face of goal and into the far corner with his left foot, quietening the home crowd who had been goading Chelsea boss Carlo Ancelotti throughout the first half.
Chelsea's confidence was growing and when a free-kick was awarded for a horrible Abdoulaye Meite hack on Drogba, there was a tangible sense of unease from the home fans as Lampard stepped up to strike. He would have netted were it not for a superb flick of the wrist from Carson, whose fingertips just about did enough to divert the ball over the bar.
But the England midfielder was to have his moment before the break. Drogba, who was limping towards the technical area to receive treatment, effortlessly stroked the ball down the left flank towards Malouda, who went into the box before picking out Lampard to slam home from the edge of the area.
That was the last act of a first half that had threatened to expose the champions' brittleness, but instead coaxed out their gritty side. The midfield trio of Lampard, Mikel and Michael Essien overpowered their counterparts, but it was Drogba who led the charge with a virtuoso performance.
Neither manager made a change at the break, but West Brom were pushing up much higher, although Morrison twice wasted chances to chip away at Chelsea's lead; first by attempting an audacious lob and then by failing to shoot on his right foot in the box.
The visitors almost stretched their lead as a Drogba cross found Lampard, but the Englishman's dive was not enough to steer it towards goal, with Kalou likewise getting too faint a touch on the ball at the back stick.
Ancelotti, with close to half an hour remaining, withdrew the visibly tiring Branislav Ivanovic for the more attack-minded Jose Bosingwa, as both Chelsea full-backs pushed forward to offer width, with the champions clearly out to add to their lead.
Attention soon started towards the Spanish striker warming up on the sidelines, who appeared eager to get into the fray, constantly checking back at the dugout to see if there was any sign that his introduction was imminent.
Before the former Liverpool man could come on, however, the Baggies made a double swap as Paul Scharner and Morrison were replaced by Carlos Vela and Gonzala Jara respectively.
The Chelsea fans continued to chant for the introduction of their £50 million man, and saw Lampard replaced by Yossi Benayoun, who wasted the chance to set up Kalou for a fourth by skewing a pass across the box behind his team-mate.
Odemwingie then came close with one of West Brom's few chances of the second period, but he could only blast wide under the pressure of Cole.
Finally Torres did come on, replacing Drogba, and he found the net within minutes, only to be ruled offside, although replays suggested that he may have been level before he latched onto a through-ball and rounded Carson.
Brunt then came close to cutting down the deficit with a whipped free-kick that only narrowly arched wide.
Torres then summed up his Chelsea career by farcically missing the ball in the box, but this was a day that belonged to the London side and boss Ancelotti, whose troops played like champions rather than overblown failures.
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