The Hammers were second best throughout the first half and were fortunate to be only 1-0 down after Juan Mata scored the first goal of Benitez's tenure, but Sam Allardyce's men were transformed after the break and levelled through Carlton Cole before half-time substitute Diame and late arrival Maiga found the net in the last five minutes.
Chelsea made the perfect start when they took the lead with only 13 minutes on the clock as Victor Moses received possession out on the right and threaded a pass through the West Ham defence for Fernando Torres, with the Spain international advancing to the goal-line before cutting the ball back into the path of Mata to clinically side-foot into the bottom corner.
The Blues could have doubled their lead when Ramires intercepted Guy Demel's sloppy pass out of defence, bursting into the box and finding Moses, but he failed to make a clean contact. There was another opportunity in the 25th minute when a swift break found two Hammers defenders facing four Chelsea players, but Ramires' pass to Torres was not the best and the striker's shot was weak and over the bar.
Kevin Nolan did have the ball in the net on a rare foray forward for West Ham but his overhead kick was ruled out with Martin Atkinson's whistle having already stopped play after James Collins was ruled to have held down Branislav Ivanovic.
Jussi Jaaskelainen was then called into action to make a superb stop from Mata, with the Spaniard's powerful shot from 12 yards after excellent work from Moses on the left expertly parried by the goalkeeper before Joey O'Brien slid in to block the follow-up.
Petr Cech escaped with only a yellow card as he strayed outside his area to punch clear above Gary Cahill and Matt Jarvis before being forced into his only real save of note on the stroke of half-time as Nolan headed goalwards after Collins' free-kick had looped up off the wall.
There was renewed purpose from West Ham at start of second half with Allardyce sending on Diame and Matt Taylor for Gary O'Neil and James Tomkins and they pressed forward with greater intensity, forcing a series of corners, with Cahill needing treatment for a bloodied nose after stooping to head clear and taking the full force of Carlton Cole's boot.
And the Hammers were back on level terms in the 63rd minute when Jarvis' cross from the left looped up into the air and Cole clambered all over the back of Ivanovic to divert his header beyond a bemused Cech at the near post, with Chelsea's players awaiting a whistle which never came and John Obi Mikel booked for his continued protests.
The Blues were inches away from restoring their advantage as Mata curled a glorious free-kick against the face of Jaaskelainen's upright, moments after the goalkeeper denied Eden Hazard before Torres showed a clear lack of conviction with his weak follow-up. Chelsea appealed for a penalty in the 71st minute when Moses went down under Taylor's challenge but Atkinson was not interested, while moments later Jaaskelainen's throw went straight to Hazard, but his attempted chip drifted over the bar and behind before he was replaced by Oscar.
Jarvis had a glorious chance to claim the points for the Hammers in the 82nd minute as he burst clear onto Mark Noble's through-ball, only for Cech to make an excellent save, with Winston Reid seeing his header cleared off the line by Ashley Cole from the resulting corner, but the Hammers were not to be denied.
In the 86th minute, Jarvis clipped the ball into the box for Carlton Cole to lay it back to Diame to drill home from 15 yards with Ivanovic offering only weak resistance, before Ashley Cole's blunder in defence allowed Maiga to play in Taylor, whose shot was parried by Cech only for the Mali forward to gobble up the rebound.