HJK's good form was ended abruptly by KuPS last time out, but bouncing back quickly would not be guaranteed against a Lahti side who were unbeaten in their previous three.
And so it proved, after the home side went ahead with two minutes of the first half to play - Mamodou Sarr rolled the ball across the edge of the box for Otso Koskinen to roll into the bottom corner with a first-time finish.
HJK turned to Teemu Pukki and Alex Ring on the hour, but not only could the visitors not generate enough chances to draw level, they fell further behind 15 minutes from time when Yohan Cassubie won possession and poked it into the path of Neemias, who took aim from 20 yards and sealed the win.
Lahti stay in eighth, just three points behind HJK, who remain in fifth, eight points off the leaders.
AC Oulu missed the chance to move within a point of the top two, as they were well beaten by TPS who returned to winning ways in fine style. The game was over as a contest by half-time - TPS were ahead nine minutes in when Albijon Muzaci's lay-off was powered in by Marius Konkkola.
The roles were reversed seven minutes later as Konkkola teed up Muzaci for his fourth goal in three games, and though he was unable to add a fifth after an hour when he missed a penalty, the points had already been secured thanks to Konkkola completing his brace in first-half added time.
Only goal difference keeps the Turku side into the bottom half, but they are now level on points with HJK. Oulu remain in fourth having taken one point from their last three matches.
Mariehamn's dishing out of three points to every opponent they face reached Helsinki, where they were beaten for the eighth league game in a row and ninth in all competitions, while Gnistan ended a run of three without a win.
Saku Ylatupa opened the scoring just six minutes in, but a penalty miss from Roman Eremenko midway through the half meant the hosts failed to extend their advantage before the break. Adeleke Akinyemi finally put daylight between the sides early in the second half, but Mariehamn wouldn't lie down, and Adam Stroud got them back in the game soon after.
A second from Akinyemi still wasn't quite enough, as Wille Nunez cut the deficit back to one once again, only for a Danny Perez strike in the 94th minute to finally settle matters and to keep Gnistan in the top half, above TPS.
