The Danish midfielder scored in the 48th minute after both teams had a player sent off and AZ Alkmaar missed a penalty in a game that started slowly before springing into life.
Ajax defender Joel Veltman was sent off in the 33rd for two cautions in as many minutes, with the second booking coming after he gave away a penalty that AZ's Aron Johannsson failed to convert.
AZ were also reduced to 10 men when Mattias Johansson was sent off two minutes into the second half.
PEC Zwolle grabbed a late winner to beat NEC Nijmegen 2-1 in Wednesday's other semi-final. The final is in Rotterdam on April 20.
- Sports & Recreation
- AZ Alkmaar
- Joel Veltman
- Mattias Johansson
- Aron Johannsson