Goals from Michail Antonio and Declan Rice secured a deserved three points for the Hammers in Croatia against the side that knocked Tottenham out of the competition last season.
David Moyes' side dominated the clash but their opener came in very fortuitous circumstances as Antonio latched onto Kevin Theophile-Catherine's woeful backpass before rounding Dominik Livakovic and poking the ball home.
There was nothing fortunate about Rice's strike though as he showed tremendous athleticism and composure on 50 minutes.
Rice started it all as he intercepted the ball on the halfway line. The midfielder ran down the left channel, shrugging off a Zagreb defender before slotting the ball through Livakovic's legs to seal the win.
Matt Holland labelled it the 'perfect performance' on talkSPORT 2 and West Ham will be making Moyes' wish come true of reaching the Europa League's latter stages if they keep that level up.
They dictated the play from the off without creating too many clear-cut chances until Antonio's goal, which came from nowehere.
Antonio was alive to things as Theophile-Catharine sold his teammate short and put West Ham in front, rounding the Zagreb goalkeeper and successfully completing the easy finish.
They could have gone 2-0 up moments later as Ryan Fredericks found himself through on goal on the counter but his shot was blocked by Livakovic.
Fredericks' shot was from an acute angle on the right but Rice had no such troubles when in on the left as he doubled West Ham's advantage.
Rice ran half the length of the pitch, ignoring the onrushing Antonio to take the shot himself, which megged Livakovic.
West Ham didn't look back after that as they showed great maturity to restrict Zagreb to half chances and see out the match.
With Genk only beating Rapid Vienna 1-0, West Ham are top of Group H after matchday one. Meanwhile, Tottenham are fighting it out in the Conference League. How times change…
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