Second-half goals by Rasmus Elm and Erik Falkenburg earned AZ a 2-0 win at Graafschap Doetinchem. Elm scored with an in-swinging corner four minutes after the interval and Falkenburg sealed the points in stoppage-time.
Alkmaar have 52 points after 29 matches, three ahead of in-form Ajax Amsterdam who beat RKC Waalwijk 3-0 to leapfrog Twente, beaten 3-1 at NEC Nijmegen, and PSV, who also lost 3-1 at NAC Breda.
Jan Vertonghen put Ajax ahead after 26 minutes when he scored at the near post, while Siem de Jong and Lorenzo Ebicilio struck in the second half to secure a fifth straight for the defending champions.
Lasse Schone converted a penalty on the stroke of half-time to put Nijmegen ahead against Twente and second half goals by Kevin Conboy and Genero Zeefuik sealed victory before Glynor Plet pulled one back.
Steve McClaren's Twente are third on 48 points with one game in hand on the top two.
PSV, also on 48, ended a bad week by slipping to defeat at Breda, three days after losing 4-2 to Valencia in the Europa League.
Georginio Wijnaldum opened the scoring for PSV after five minutes but Santi Kolk and Anthony Lurling quickly replied.
PSV defender Stanislav Manolev was sent off just before half-time and the hosts, who added a third goal through Kolk, were also reduced to 10 men just past the hour when Robert Schilder was dismissed.
