West Ham stunned Liverpool as they secured a huge victory that moved them above their opponents into third in the Premier League and dented the Reds' title hopes.
Liverpool went into this game looking to set a new club record of 26 games unbeaten, as well as get a win that would move them up to second and within a point of leaders Chelsea.
But they fell behind early on when Alisson, under pressure from Angelo Ogbonna, flapped at Pablo Fornals' corner and succeeded only in helping the ball into his own goal.
Both sides were then fortunate not to be reduced to 10 men in a scrappy first half, with Aaron Cresswell catching Jordan Henderson with a heavy challenge before Ogbonna was hit in the face by a stray Diogo Jota elbow that resulted in the West Ham defender having to be substituted.
But a moment of quality four minutes before half-time brought Liverpool level when Trent Alexander-Arnold curled home a stunning strike after playing a short free-kick to Mohamed Salah.
Craig Dawson hit the woodwork with a header from a corner for West Ham after the break before Lukasz Fabianski superbly kept out Sadio Mane's fierce volley from close range.
But West Ham regained the lead midway through the second half when Fornals was slipped through on goal and Alisson was unable to keep out his low shot.
That goal led to the London Stadium exploding into deafening noise, which became even louder when Kurt Zouma headed in a corner minutes later.
Divock Origi swept in a second on the turn to give Liverpool hope of rescuing something, but West Ham kept their nerve to take the three points.
The Hammers are third, level on 23 points with second-place Manchester City and three points behind leaders Chelsea. Liverpool, meanwhile, drop to fourth on 22 points.