Ajax restored its three-point advantage at the top of the Eredivisie on Sunday with a 4-0 home victory against Heracles.
Wins for PSV Eindhoven and Vitesse on Saturday left Ajax in a three-way tie atop the league.
But the club collected its fifth successive victory on Sunday despite playing most of the second half with 10 men.
Lasse Schone's goal after 13 minutes had Ajax up 1-0 at halftime, but goalkeeper Kenneth Vermeer was sent off in the early stages of the second half, although the home side only got better.
Just four minutes after the red card, Kolbeinn Sigthorsson doubled the lead for Ajax, while Ryan Babel and Christian Eriksen each added another goal for the home side in the final 15 minutes.
Utrecht downed Den Haag, 1-0, with a second-half goal from Jens Toornstra, NEC and AZ Alkmaar played to a 1-1 draw despite NEC playing the final half hour with 10 men and Groningen toppled Heerenveen, 3-1, with Michael de Leeuw scoring his ninth goal of the campaign for the victors.