Thomas Tuchel admitted Chelsea paid the price for too many "individual mistakes" as Arthur Masuaku's fortuitous late winner gave West Ham a shock 3-2 victory on Saturday.
Tuchel's side went ahead through Thiago Silva but Manuel Lanzini's penalty drew West Ham level at the London Stadium.
Mason Mount restored Chelsea's advantage just before half-time, only for Jarrod Bowen to blast West Ham's second equaliser soon after the interval.
Chelsea dominated for long periods but made too many mistakes at both ends.
Even so, it was rough justice to leave empty-handed after Masuaku's miscued cross somehow found its way past Edouard Mendy with three minutes left.
"We need to admit we did too many crucial mistakes at the moment. That cost us points against Manchester United and today as well," Tuchel said.
"If you want a result at this level you have to minimise mistakes. We talked about it before, it's obviously not helped."
Senegal keeper Mendy was partially at fault for two of the goals and Tuchel said: "The decision making from Edouard is not the best in this moment."
It was the Blues' first defeat in their last 13 games in all competitions and their spell at the top of the Premier League might be over by Saturday evening.
They are just one point ahead of second placed Manchester City, who visit Watford later on Saturday, with third placed Liverpool also able to go above Chelsea if they win at Wolves.
After admitting Chelsea rode their luck to beat Watford in midweek, Tuchel will be concerned at another disjointed display from the European champions.
"Their third goal is a strange one but we could not finish our chances. We had to be more ruthless," Tuchel said.
"Every loss is a bump in the road. We need to play more accurate, more stable and reduce big mistakes."
Fourth placed West Ham once again showed their ability to make life uncomfortable for the big guns.
The Hammers had already ended Liverpool's long unbeaten run this season, as well as knocking Manchester City and Manchester United out of the League Cup.
Now Chelsea have joined the list of star-studded teams driven to distraction by David Moyes' well-drilled side.
"Truthfully, I didn't think we played that well, but to get those goals against a team that don't concede many is a great credit to the lads," Moyes said.
"I said to Arthur I thought it was a great cross! It was very fortunate, but in football you need that sometimes."
- Chelsea collapse -
Chelsea's early dominance was rewarded in the 29th minute when Silva met Mason Mount's corner with a downwards header that Lukasz Fabianski could only push into the net via the far post.
But their concentration wavered five minutes before half-time when Jorginho played a suicidal backpass that Mendy scrambled to reach before Bowen.
As Mendy tried to dribble around Bowen, he lost control of the ball and sent the West Ham forward crashing to the turf with a panicked tackle.
It was a comedy of errors and Lanzini delivered the punchline as he drove the penalty into the roof of the net.
Chelsea's response was instant as a sweeping move put them back in front four minutes later.
Hakim Ziyech's majestic long pass picked out Mount, who ran in behind Ben Johnson to caress a sublime volley past Fabianski from just inside the area.
Missing the injured N'Golo Kante as the midfield shield in front of them, there was a lack of cohesion about Chelsea's defence and they surrendered a 56th minute equaliser.
Vladimir Coufal was able to poke a pass to Bowen on the edge of the area as Chelsea fatally backed off and the forward drilled a superb low strike into the far corner.
There was worse to come for Chelsea in the 87th minute.
When Masuaku took possession and shaped to cross, there seemed little danger, but the defender's delivery suddenly swerved in mid-air, deceiving Mendy as it arched past the stunned keeper.