An impressive recovery saw the Hammers overturn Juan Mata´s first-half opener with goals from Carlton Cole, Mohamed Diame and Modibo Maiga.[break]
It left Chelsea without a win in seven Premier League games -- their worst run since February 1995.
Benitez has been in charge of three of those games, but after successive scoreless draws, the manager was entitled to feel confident he would oversee his first victory when Mata´s goal capped a dominant first-half display.
But West Ham were transformed after manager Sam Allardyce introduced Diame and Matt Taylor at half time and ultimately deserved victory for a vastly improved performance in the second period.
Benitez made four changes to the side that started the 0-0 draw against Fulham, underlining the manager´s pre-match assertion that the demands of the season are already beginning to tell on Chelsea´s senior players.
Mata, surprisingly held back on the bench in mid-week, returned in place of Ryan Bertrand, while Victor Moses and John Obi Mikel came in for Oscar and Oriol Romeu, and Gary Cahill stepped in for suspended centre-back David Luiz.
Allardyce was forced into changes of his own, after striker Andy Carroll was ruled out for two months with a knee injury picked up in the narrow mid-week defeat at Manchester United.
After the frustrations of Benitez´s first few days in charge at Stamford Bridge, Chelsea desperately wanted to make a positive start.
And after surviving an early lapse when Winston Reid was allowed a free header in the Blues penalty area, they opened up the West Ham defence with ease to allow Mata to score the confidence-lifting opening goal.
The move was as simple as it was incisive, stemming from a throw-in on the half-way line.
Moses released Torres on the right-hand side of the West Ham area and the striker´s cut-back found Mata, who finished side-footed from 12 yards.
The effect on Chelsea was tangible and Benitez´s side began to press forward with a fluency that was noticeably absent during their previous two games.
Ramires found space on the left to set up Moses, who shot wide, before a sweeping four-man break ended with Torres firing wastefully over.
Chelsea´s failings meant West Ham remained in the game but Allardyce´s side appeared to be affected by the demands of a testing week that had seen them lose at Tottenham Hotspur and Old Trafford.
They had Jussi Jaaskelainen to thank for restricting Chelsea to a single-goal lead at half-time after the goalkeeper produced an excellent save to repel Mata´s close-range shot.
Allardyce made two changes at half-time, introducing Diame and Taylor for James Tomkins and Gary O´Neil.
And the changes had an immediate effect, with the Hammers transformed from the side that had struggled in the first half.
Immediately, Chelsea were pressed onto the back foot and forced to defend with increasing desperation.
But their back-line was broken when Cole out-jumped Branislav Ivanovic to head the home side level in the 63rd minute.
Chelsea responded positively, Mata hitting the post with a curling free-kick.
But the momentum was with West Ham and Allardyce´s side went ahead in the 86th minute when Diame latched onto Cole´s lay-off to drill the ball home.
And substitute Maiga sealed the points when he found the roof of the net after Taylor´s shot had been parried by Cech.
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