Leandro Bacuna's second-half penalty earned FC Groningen a 1-0 victory over ten-man Ajax at the Euroborg, condemning the champions to a first Eredivisie defeat since March.
The 20-year-old midfielder coolly converted from the spot in the 62nd minute to further punish the Amsterdammers after Gregory van der Wiel had been sent off for handling the ball. The result saw Ajax slip six points behind leaders AZ Alkmaar, who were 3-1 winners at VVV-Venlo.
The hosts almost opened the scoring in the seventh minute with Virgil van Dijk heading just wide from Dusan Tadic's corner. Miralem Sulejmani went close at the other end moments later, bursting through the Groningen defence before fizzing a left-footed shot wide.
Jan Vertonghen then had a fierce drive beaten away by Groningen goalkeeper Brian van Loo just after the half-hour mark, and Ajax keeper Kenneth Vermeer also had to be alert two minutes before the break to keep out a 25-yard blast from Maikel Kieftenbeld.
Theo Janssen fired a free-kick over the bar for Ajax and Bacuna headed narrowly over for Groningen as half-time approached, but the teams went in at the interval goalless.
Lorenzo Burnet came to the hosts' rescue in the early stages of the second half, clearing off the line twice in quick succession to preserve his team's clean sheet.
Ajax substitute Dmitri Bulykin brought a fine save out of Van Loo in the 58th minute following a purposeful run down the right, but within five minutes the match had turned against Frank de Boer's side.
Van der Wiel, who had been booked in the first half for a foul on Tadic, was given his marching orders for handling in the area and Bacuna placed the resulting spot-kick into the bottom-left corner of Vermeer's net.
Ajax's best chance to equaliser came in the 76th minute through a Siem de Jong header, but it was brilliantly saved by Van Loo. Groningen could have extended their lead four minutes from time, but David Texeira failed to make a meaningful connection with Tadic's cross.