The leaders, searching for their maiden league title, had previously seen their lead extend to double digits, but only when the chasing pack had a game in hand.
Mjallby did their part in changing that when they travelled to Degerfors, and they took the lead in the 13th minute, when a Jeppe Kjaer free-kick was whipped in from the right, and Tom Pettersson got in front of the goalkeeper to prod home.
Having picked up a surprise point away at Malmo last week, Degerfors failed to have a shot on target in response, meaning one goal was enough for Mjallby to extend their unbeaten league run to 15 matches, while Degerfors are 14 without a win and remain in the bottom two.
That victory put the pressure on Hammarby, who failed to keep pace with the team above them by throwing away a late lead to draw 3-3 in a dramatic Stockholm derby with Djurgarden.
The Bajen seemed to be dealing well with the pressure, as they were 2-0 up within 16 minutes. First, Djurgarden made a mess of playing the ball out from the back and presented a perfect chance for Markus Karlsson. Then, Nahir Besara scored a stunning free-kick from 30 yards out.
Midway through the first half, it was Hammarby's turn to give the ball away in a dangerous area, and Djurgarden capitalised through Jeppe Okkels' deflected strike.
The home side completed the comeback 10 minutes after break when a deep corner was headed back across goal before Adam Stahl bundled it over the line. However, Hammarby went back in front on 68 minutes when Montader Madjed turned home Paulos Abraham's fierce ball across.
But on 96 minutes, Djurgarden revelled in denting their 3Arena rivals' title hopes, when Stahl again popped up at a corner to make it 3-3 with the last kick of the game. The Iron Stoves stay eighth despite going eight unbeaten, while it's just one win in four for Hammarby.
Hammarby stay six points clear of third place for now with AIK not in action until tonight when they meet Brommapojkarna, and champions Malmo dropping more points with a 2-2 draw with Elfsborg.
It's a third draw in succession for MFF, while Elfsborg continue to drop away from the European race as they have taken just two points from their last five.
They at least did well to salvage a point, having been 2-0 down at half-time after two Daniel Tristan Gudjohnsen strikes, before two goals in six second-half minutes from Besfort Zeneli and Rasmus Wikstrom earned a point.
Goals from Isak Bjerkebo, Joakim Persson and Victor Svensson gave Sirius a 3-0 win over GAIS which means Mjallby are the only team in the top five to pick up a win so far this round.
The Gothenburg side continue their inconsistent form which has seen them follow a win with defeat for over a month, while it's now four wins in their last five for Sirius, who have climbed to 10th place.
Oster remain in the relegation playoff place but are now just one point from safety after they won a thriller 4-3 at home against bottom-of-the-table Varnamo.
The hosts were 3-2 down as the clock struck 90 minutes after a Kai Meriluto hat-trick looked to have been enough to end Varnamo's dismal run, but a close-range finish from Ivan Kricak and a drilled Niklas Soderberg finish turned it around at the death for Oster.
For Oster, it's a second win in four, and they leave Varnamo nine points from safety and on a three-match losing run.
Finally, Norrkoping and Halmstad kept themselves hovering above Oster and the relegation zone with a 1-1 draw, in which Norrkoping's Arnor Traustason cancelled out Rocco Ascone's first-half penalty for the hosts.