PSV have 46 points from 21 matches, four ahead of second-placed Twente Enschede who are at home to Utrecht on Sunday.
Ajax Amsterdam, third on 40 points, travel to VVV Venlo on Sunday.
Wijnaldum began the goal feast after nine minutes when he finished off good work from Atiba Hutchinson.
The home team then had to wait until the 57th minute before an own goal from Christian Supusepa doubled the lead.
Mertens curled a free kick round the wall to make it 3-0 and after Jeremain Lens and Juergen Locadia scored the fourth and fifth, the Belgian international added his second of the game with a superb diagonal drive from 25 metres.
Wijnaldum completed the rout of eighth-placed Den Haag in stoppage time with his 11th league goal of the season.
Ten-man AZ Alkmaar suffered a 1-0 home defeat by Groningen after a 27th-minute goal from Andraz Kirm.
Alkmaar's Viktor Elm was sent off after 65 minutes for a wild tackle.
Roda JC Kerkrade and Heracles Almelo shared the points in a six-goal thriller, an injury time own goal from Christian Dorda earning Roda a point.
Lowly NAC Breda registered their third straight win by beating 10-man PEC Zwolle 3-0.
- Sports & Recreation
- Jeremain Lens