Demarai Gray atoned for a first-half penalty miss by scoring the winning goal as a second-string Leicester piled the pressure on West Ham manager Manuel Pellegrini with a 2-1 victory at the London Stadium on Sunday.
A fourth successive home defeat for the Hammers, who have won just twice in 14 matches, left them a point above the Premier League relegation zone and to make matters worse came the defeat came against an under-strength Leicester.
Foxes boss Brendan Rodgers made nine changes to the side hammered 4-0 by leaders Liverpool on Thursday, a decision that was as much a response to English football's traditionally packed Christmas and New Year programme as it was a chastening defeat.
Gray, one of the new faces, saw Lukasz Fabianski save his early spot-kick but the winger made amends after the interval to fire second-placed Leicester to a first league win in four games that reduced Liverpool's lead at the top to 10 points.
The Reds, however, will again go 13 points clear if they beat Wolves on Sunday.
"I was fine taking the penalty, I'm not really nervous in that situation," said Gray.
"Those things happen but it was important to react properly and I did and I got the goal and I was happy."
Pellegrini dropped Mark Noble to the bench following the Hammers captain's on-field row with Angelo Ogbonna at Crystal Palace on Boxing Day, while the Italian defender was not even named in the squad.
West Ham's slide down the table has coincided with Fabianksi spending three months out injured.
But the Poland international made a bad start to his return, his first act of the game coming when he bundled over Kelechi Iheanacho to concede a penalty.
Leicester were without star striker and regular penalty-taker Jamie Vardy, the England international missing the match as his wife Rebekah gave birth to a girl.
Gray took over the spot-kick duties in Vardy's absence responsibility only to see Fabianksi make a fine save low to the keeper's left.
The Hammers, however, did not capitalise on their reprieve and it was Leicester who went ahead five minutes before half-time when Ayoze Perez headed James Justin's cross to the far post back to Iheanacho, who stooped to nod in from close range.
The goal stung West Ham into a response and they were level before half-time, with Brazilian winger Felipe Anderson sharing a one-two with Ryan Fredericks before crossing low for Pablo Fornals toscore his first Premier League goal.
Leicester, however, regained the lead 11 minutes into the second half after Perez's through-ball pierced the home defence with Gray curling a fine shot inside the near post after taking a touch.
This time, however, West Ham could not muster a response.
(For more sports news and updates, follow Ahram Online Sports on Twitter at @AO_Sports and on Facebook at AhramOnlineSports.)