Former Chelsea player Emerson Palmieri scored against his former club to earn West Ham a point and increase the pressure on Blues boss Graham Potter.
Chelsea fielded a new-look £200m forward line but, despite taking the lead, were once again frustrated and have won just once in seven Premier League matches.
Joao Felix had already had a goal ruled out for offside when the Portugal forward fired visitors Chelsea ahead with a volleyed finish following a delicious assist by Enzo Fernandez, the £107m British transfer-record signing from Benfica.
Kai Havertz thought he had doubled the lead only for Chelsea’s celebrations to be cut short by another offside flag, before West Ham equalised through Italy international Emerson’s first Premier League goal.
The 28-year-old, who spent four years at Stamford Bridge, finished at the far post after Vladimir Coufal’s cross was flicked on by Jarrod Bowen.
Chelsea, who have dropped 14 points in the last seven top-flight matches, carved out the better chances but could not make them count.
Noni Madueke was lively on his first start for the club since his £30.7m arrival from PSV Eindhoven and the 20-year-old forced a fine save from Lukasz Fabianski before the West Ham keeper kept out a free-kick by Felix.
In a dramatic finish, West Ham thought they had won it in the closing stages when substitute Tomas Soucek scored from close range but it was ruled out for offside – the third disallowed goal of the match.
This was another hugely frustrating day for Potter and Chelsea as strong appeals for an 89th-minute penalty were waved away after Soucek appeared to handle Conor Gallagher’s shot inside his own box.
