A Vladimir Coufal own goal via Savinho’s shot had given Pep Guardiola’s side a 10th minute lead at the Etihad.
Erling Haaland then powered home a header from Savinho’s inviting cross to double City’s advantage before the break.
Savinho was the architect once again after the restart to set up Haaland’s brace with a delicious through ball.
City then further extended our lead through Phil Foden before a late Niklas Fullkrug effort gave the Hammers a modicum of consolation.
It made it the perfect start to the New Year on an afternoon that also saw Kevin De Bruyne chalk up his 400th City career appearance.
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