Rafa Benitez's decision to rotate his Chelsea side backfired as bottom-place Queens Park Rangers produced its first topflight win at Stamford Bridge since 1979 on Wednesday in one of the biggest shocks of the season.
Compounding Chelsea's misery, it was former winger Shaun Wright-Phillips whose goal in the 78th minute halted a four-match winning streak by clinching QPR's 1-0 west London derby win.
"We were tired and weren't precise," Benitez said. "We made a mistake and gave them their chance."
And Chelsea missed a chance to rise from fourth place where it is 14 points behind leader Manchester United with a game in hand.
Given that QPR had capitulated to Liverpool in a 3-0 loss on Sunday, Benitez felt confident enough to start key players, including Eden Hazard and Juan Mata, on the bench.
And despite then losing to the league's worst team, Benitez defended his tinkering.
"I don't regret the changes because we can't carry on with the same players in every game," said Benitez, whose appointment in November was unpopular with many fans. "If you're playing a team at the bottom of the table at home, you have to trust your players."
Yet, the Chelsea players will go down as the first to lose to QPR in the league at home since April 1983 when they were playing in the second tier.
Not since March 1979 had Chelsea lost a west London derby in the top division, and QPR's memorable victory could spur its bid for Premier League survival.
Harry Redknapp's side is still in a perilous position at the bottom but is now only behind Reading on goal difference and five points from safety.
"I can't remember many better wins than this one, with Chelsea flying at the moment," Redknapp said.
Not only was this QPR's first away win of the season but also the team's first on the road since November 2011 — when Neil Warnock was in charge, two managerial appointments ago.
"They've set the standard what they've got to do if we are going to get out of this mess," Redknapp said.
It could have been a more comfortable night for QPR had Chelsea been reduced to 10 men in the fourth minute.
Midfielder Marko Marin, who started for Chelsea despite barely 20 minutes of topflight experience in England, was fortunate to receive just a yellow card when he launched a studs-up challenge over the ball and into Stephane Mbia's shin.
While Marin was one of the unexpected Chelsea starters, Benitez didn't use this game to carry out his plan to rest striker Fernando Torres, who has started every league match this season.
Despite rediscovering his scoring touch since Benitez arrived in November, Torres went into the QPR game without finding the net in the past two games after hitting seven goals in the previous six matches.
And the Spain forward rarely looked like breaching Julio Cesar's goal on Wednesday — despite knowing that Demba Ba is soon due to arrive from Newcastle to provide competition up front.
There was a tame low shot in the 39th minute that Cesar easily collected on a frustrating night for Torres in which he struggled to find space.
Throughout the first half, Chelsea struggled to break down its resilient visitors, with David Luiz shooting over, Oscar's shot blocked by Cesar's legs and Victor Moses firing over.
"Our keeper was different class tonight," Redknapp said. "He gave us the feeling he was never going to get beat."
But the intensity from Chelsea stepped up immediately at the start of the second half.
An inviting cross from Marin was poked wide by Moses, Branislav Ivanovic headed over from a corner, and Torres was denied with a first-time strike from point-blank range.
To inject more creativity, Hazard took Marin's place on the hour, but QPR was creating moments of danger.
Jamie Mackie's run on goal was snuffed out by Cahill, before Shaun Derry's header at far post from a corner was collected by goalkeeper Ross Turnbull.
Moses came close when a shot spun over from distance before being replaced by Mata, and Frank Lampard had a goal ruled out for offside.
But it was QPR which took the lead after Chelsea had failed to properly clear a corner, and Adel Taarabt teed up the winner for Wright-Phillips.
The Englishman, who replaced the injured Junior Hoilett early in the first half, was at Chelsea between 2005 and 2008 before returning to Manchester City, which he left to join QPR in 2011.
He hadn't score in the league since May 2010.
"I learnt playing football there are lots of ups and downs," the 31-year-old Wright-Phillips said. "It plays on your mind when you don't score."
Scores from the other Wednesday matches
Chelsea 0, Queens Park Rangers 1
Liverpool 3, Sunderland 0
Newcastle United 1, Everton 2