Champions Twente thrashed at Heerenveen
PSV Eindhoven, held to a 0-0 draw at Graafschap Doetinchem on Saturday, are top with 38 points from 18 matches.
Twente are second, one point behind, while Groningen are two points off the pace after beating AZ Alkmaar 2-0 on Sunday.
Assaidi put Heerenveen ahead after 20 minutes and restored the lead before the interval with a well-placed shot after Nacer Chadli had equalised from close range.
Chadli added his second in the 51st minute but the home team then took control with goals from Viktor Elm, Luciano Narsingh, Assaidi and Mika Vaeyrynen (penalty).
In Groningen, Tim Matavz surprised Alkmaar goalkeeper Sergio Romero at the near post to put the hosts ahead after 11 minutes and later struck his second of the match in injury time.
Groningen's Tim Sparv was sent off 13 minutes from time after receiving a straight red card. Alkmaar's Hector Moreno was also dismissed 12 minutes later.
Ajax Amsterdam remained fourth with 35 points following a 1-0 win at Vitesse Arnhem. Christian Eriksen scored the only goal in the 11th minute to earn coach Frank de Boer victory in his first league match in charge.
Feyenoord climbed to 13th by beating neighbours Excelsior Rotterdam 1-0, Luc Castaignos, 18, scoring his sixth goal of the season midway through the first half.