A goal from former player Dimitar Berbatov has seen Tottenham Hotspur sink to a third straight defeat, as they lost 1-0 to a Fulham side that earned a rare away win in the English Premier League.
Victory would have moved Andre Villas-Boas' side to within two points of second-place Manchester City, but they were unable to find a way past Fulham's excellent goalkeeper, Mark Schwarzer.
Tottenham now find themselves just four points ahead of north London rivals Arsenal, who have a game in hand, in the scramble for Champions League qualification places.
Berbatov's goal turned out to be just enough for Fulham - managed by former Tottenham coach Martin Jol - to taste victory away from Craven Cottage for only the third time in league action this season.
It was no surprise that Gareth Bale was restored to the Tottenham starting line-up after suspension prevented him from playing at Inter Milan in mid-week, when Spurs reached the Europa League quarter-finals despite a 4-1 loss.
Emmanuel Adebayor, whose extra-time goal in Milan squeezed Tottenham into the last eight on away goals, kept his place in the side, with Jermain Defoe dropping to the bench.
Bale made a lively start, with Fulham captain Brede Hangeland stopping his first foray into the box at the expense of a corner before the Tottenham man curled a long-range effort over.
But Jol's side worked hard to limit Tottenham opportunities, with Sigurdsson then scuffing a half-chance wide, and a lack of atmosphere in the home sections of the ground reflected that.
Tottenham made a change at the break, with centre-back Michael Dawson, who had suffered a hamstring injury, replaced by ex-Fulham man Clint Dempsey.
The American moved to the right of midfield, Benoit Assou-Ekotto switched to left-back, and Jan Vertonghen joined Steven Caulker in the middle of defence.
It livened up the game immediately and Bale forced Schwarzer into his first taxing save of the game with a long-range attempt.
Fulham's reply was a 52nd-minute opening goal on the counter-attack, with Berbatov arriving in the six-yard box to steer Riether's low delivery from the right past goalkeeper Hugo Lloris.
It was the first time the Bulgarian had scored at White Hart Lane since Tottenham sold him to Manchester United in August 2008, and he did not celebrate.
Defoe came on for Sigurdsson just after the hour mark, which saw Bale switch to the right of midfield, and Mousa Dembele was replaced in central midfield by Tom Carroll a few minutes later.
Schwarzer pulled off an admirable save to deny Defoe, who shot first-time on the turn after Caulker had set him up, and the goalkeeper was then quick off his line to get to Carroll's through-ball ahead of Bale.
The Socceroos stopper then made another fine save to deny Defoe once more after the England international met Bale's cross from the right with another well-struck first-time effort.