PSV Eindhoven moved to the top of the Eredivisie with a 4-0 home win over Heracles on Saturday night as Ajax's unbeaten league run ended.
With FC Twente not in action until Sunday, when they tackle Feyenoord, PSV seized their chance to move two points clear.
Dries Mertens gave the hosts a 19th-minute lead and Luciano Narsingh added a second goal in the 36th minute.
Kevin Strootman stretched the cushion to 3-0 after 74 minutes, shortly before Heracles had Kwame Quansah sent off, and substitute Tim Matavz grabbed the fourth goal in the 81st minute.
Ajax were the last undefeated team in the Dutch top flight, but that bare fact masked that their record showed only four wins from 10 games, the others all being drawn.
The Amsterdam giants remain sixth in the table, now nine points behind PSV, after a 2-0 loss at home against third-placed Vitesse, for whom Wilfried Bony scored in the 38th and 46th minutes.
Ajax head to England in the coming week to play Manchester City in the Champions League, having beaten the big-spending Premier League champions at the Amsterdam ArenA.
FC Utrecht stand fourth in the table after a 5-1 win away to second-bottom Willem II.
Cedric van der Gun's 13th-minute opener for Utrecht was countered by an own goal from Jacob Mulenga, but the visitors took charge from the moment Mulenga scored at the right end in the 44th minute to restore their lead.
Substitute Alexander Gerndt netted in the 63rd and 75th minutes, before Mulenga netted for the third time on the night, his second for Utrecht, with six minutes remaining.