Chelsea old boy Shaun Wright-Phillips stunned his former club with a 78th-minute winner as bottom side Queens Park Rangers shocked the European champions 1-0 in the Premier League on Wednesday.
The 31-year-old drove home from the edge of the box at Stamford Bridge to move Harry Redknapp's side to within five points of safety and prevent Chelsea from reclaiming third place from Tottenham Hotspur.
"They worked their socks off and they got their rewards. We deserved it and it was a fantastic performance," said QPR manager Redknapp.
"They have set the standard tonight and to get out of this mess, they have to do it every week."
Newcastle United had announced earlier in the day that they had accepted a bid for striker Demba Ba from Chelsea and last season's Champions League winners looked like they needed the extra fire-power against their London rivals.
Interim coach Rafael Benitez made five changes to his starting line-up, with Ashley Cole, Ramires, Juan Mata, Eden Hazard and the injured Petr Cech dropping out, and the changes were to prove costly.
German winger Marko Marin made his first league start for Chelsea but his only contribution of note was an ugly fourth-minute challenge on Stephane Mbia for which he was lucky to escape with only a caution.
Marin did tee up Victor Moses to shoot wide from close range early in the second half, while Julio Cesar saved brilliantly from Fernando Torres and Frank Lampard had a goal ruled out for offside, but Chelsea were below-par.
Punishment arrived with 12 minutes to play, when Adel Taarabt teed up Wright-Phillips to find the bottom-left corner from 20 yards and bring Chelsea's four-game winning run to a juddering halt.
"A lot of things went wrong today," said Benitez.
"We knew we were playing against the bottom-of-the-table side and we spoke about that. We knew we had to be focused but we did not have the tempo and some players may have been tired.
"It was going wrong against a side who played deep and were organised."
Meanwhile, Everton bolstered their bid for Champions League football by coming from behind to win 2-1 at Newcastle.
Papiss Cisse did his best to prove there is life after his Senegal team-mate Ba by heading Newcastle into a second-minute lead after goalkeeper Tim Krul's long kick was allowed to bounce on the edge of the visitors' box.
Leighton Baines produced a goal-of-the-season contender to equalise, however, rippling the net with a magnificent 30-yard free-kick that left Krul grasping at thin air.
Substitute Victor Anichebe completed the comeback on the hour, tapping in Nikica Jelavic's low cross barely ninety seconds after coming on to send Everton up to fifth.
Liverpool reinvigorated their push for European football by sweeping Sunderland aside 3-0 at Anfield to climb two places to eighth.
Teenage winger Raheem Sterling opened the scoring in the 19th minute, galloping onto a pass from Luis Suarez and calmly lobbing the advancing Simon Mignolet.
Suarez added a second in the 26th minute, shrugging off Carlos Cuellar before beating Mignolet at his near post, and the Uruguayan made it 3-0 from Steven Gerrard's pass seven minutes into the second half.
Suarez's brace took his tally of league goals to 15, while Liverpool are now only seven points beneath the Champions League places, although fourth-place Chelsea retain a game in hand.
Ahead of the weekend's FA Cup fixtures, Manchester United lead the table by seven points after winning 4-0 at Wigan Athletic on New Year's Day, while Manchester City remain second following their 3-0 win at home to Stoke City.