Hacken's surprise defeat to Orgryte 24 hours earlier gave Hammarby the chance to take the "best of the rest" position outright, and it's a chance they took with a second win in a row following a poor run of form.
They took the lead inside 10 minutes when a Noah Persson centre from the left caused havoc in the Kalmar box, before it fell to Paulos Abraham, who lifted it over the goalkeeper for his seventh goal of the season.
Bajen extended their lead five minutes into the second half, when Montader Madjed raced down the right and whipped in a low cross which Aboubacar Keita powered into his own net.
The home side were unable to take late opportunities to put icing on the cake, but they had done more than enough to maintain Kalmar's run of not picking up a single point on the road, despite only losing once at home, on the opening day. They remain in 12th place.
Having thrown away their lead late on last time out against Mjallby, Sirius held firm this time to hold off a second-half comeback from Brommapojkarna to stay unbeaten and nine points clear at the top.
They went ahead in the 18th minute in spectacular fashion - a corner was punched away by the BP goalkeeper to Isak Bjerkebo, who took one touch to set himself before taking aim, gliding the ball into the top corner from 25 yards.
Sirius' second in the 40th minute was all of the home side's own making, as Oskar Cotton was too slow to clear the ball from his own box, allowing former Bromma Boy Oscar Krusnell to charge him down and pick out Robbie Ure, who didn't need a clean strike to score his league-high 12th of the season.
Early in the second half, a deep Mads Hansen corner was nodded home by Andreas Troelsen to reduce the deficit, but the Uppsala side comfortably held on for their 10th win of the campaign, while BP slip to 11th place having gone three without a win.
Malmo have made it three wins in a row and two out of two under Gaute Helstrup, thrashing Goteborg to move up to fifth, just a point off the European places, while the Gothenburg side remain in the relegation play-off place following successive defeats.
It was thanks to a combination of goalkeeper Robin Olsen and the crossbar that Malmo weren't behind in the first four minutes, but once Otto Rosengren's corner was headed in by Andrej Djuric in the ninth minute, there was no looking back for MFF.
Erik Botheim tucked home the second on the rebound midway through the half after Sead Haksabanovic's shot was parried, and Botheim made it a brace for himself in first-half added time, tapping home after great work at the byline by Oscar Sjostrand.
The rout was completed early in the second half when Anton Hoog raced away to score his first in Malmo colours after being played through by Botheim.
Malmo are behind GAIS on goal difference after the Mackerels beat Elfsborg by one goal, as they go unbeaten in three, while the Boras side are now a point behind in sixth having gone six without a win.
Neither side were able to make the most of decent first-half opportunities, only for Robert Thorkelsson to stamp his authority on the game six minutes into the second half, controlling Rasmus Niklasson's cross superbly before poking it past the goalkeeper.
Three wins in their last four have taken Vasteras to seventh in the table, while Degerfors' winless run has stretched to eight games, but still they stay above the bottom three on goal difference.
A foul by goalkeeper Matvei Igonen on Marcus Baggesen allowed Mikkel Ladefoged to strike Vasteras ahead from the penalty spot with only seven minutes gone, before Baggesen's deflected effort following a short free-kick from Ismet Lushaku settled the match in the 54th minute.