Hammarby only had themselves to blame for falling behind after half an hour in Gothenburg - sloppy passing at the back allowed Ibrahim Diabate to steal in and roll home the opening goal.
The visitors drew themselves level early in the second half when Abdelrahman Boudah rose highest to head in Sebastian Tounekti's cross, only for GAIS to restore the advantage on the hour mark, when Diabate scored from the penalty spot following a handball by Oskar Agren.
Diabate capped off a memorable afternoon by setting up Amin Boudri inside the final 10 minutes - he cut inside a steered a cool finish into the bottom corner.
A clumsy challenge from Victor Eriksson late on allowed Jusef Erabi to reduce the deficit in the final minutes and make it 3-2, but it wasn't enough to prevent Hammarby's four-match winning run coming to an end, as GAIS' unbeaten run reaches eight matches.
Mjallby were clearly eager to take advantage 24 hours later as they travelled to face Sirius, wasting no time in building a healthy lead.
Seven minutes in, Elliot Stroud was given too much time inside the area, and though the angle was against him, he managed to squeeze the ball past goalkeeper Ismael Diawara.
Nine minutes later, Stroud turned provider after his initial effort was saved, he squared the rebound to Jacob Bergstrom, who tucked the ball past an out-of-position Diawara.
2-0 was how it stayed until 10 minutes from the end, when a free-kick was knocked back across goal to Jakob Voelkerling, who bundled it in from close range.
However, Sirius were unable to complete the comeback and fell to a second consecutive defeat, meaning Mjallby go four points clear at the summit, with their current unbeaten run matching that of GAIS - eight matches.
AIK have reclaimed third place from Elfsborg and moved to within a point of Hammarby thanks to a 3-0 win over hapless Degerfors, who lost for the sixth match in succession.
Andronikos Kakoullis ended a two-month wait for a goal by scoring either side of half-time, before Bersant Celina put the icing on the cake in injury time.
The biggest shock of the round came in Varnamo, where the home side finally registered their first win of the season at the 15th attempt, beating Djurgarden 1-0.
To add to the drama, the goal came in the 94th minute of the match - Mohammad Alsalkhadi played a delightful one-two inside the area with Marcus Antonsson before steering the ball into the bottom corner, to inflict defeat on the same side who put five past Degerfors last week.
Both Hacken and Brommapojkarna profited from Djurgarden's slip up by going above them in the table to eighth and ninth, respectively.
Hacken did so with a comprehensive 4-1 victory over Halmstad. Adrian Svanback, Sanders Ngabo and an Amor Layouni penalty put the hosts 3-0 up before the break, then Isak Brusberg added a fourth within three minutes of the restart to secure back-to-back wins.
Malte Persson notched a consolation in the 93rd minute for Halmstad, who sit 11th after a fifth defeat in seven.
Brommapojkarna have picked up three successive wins for the first time this season, as they saw off Oster at home, 2-0. Adam Jakobsen and Victor Lind found the net in the second half to condemn Oster to successive losses and another week in the bottom two.
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