Vitesse Arnhem are third on 54 points following a 4-0 rout of ADO Den Haag.
Belgian international Mertens converted a sixth-minute penalty for PSV before setting up the second goal in the 69th minute for Wijnaldum who scored with a delicate chip over keeper Jeroen Zoet.
"It looked comfortable but we should have made the game safe earlier because we created enough chances," coach Dick Advocaat told reporters.
"I was not satisfied with [forwards] Tim Matavz and Ola Toivonen. But in the case of Toivonen it is just match fitness as he has been out for several months and only returned today."
The league's top scorer, Ivorian Wilfried Bony, took his goal tally this season to 26 with a first-half double as Vitesse stayed in the title race.
Bony also set up Marco van Ginkel's goal 10 minutes into the second half before Mike Havenaar sealed the win.
Marco van Basten's Heerenveen also beat NEC Nijmegen 3-1 thanks to two goals from Filip Djuricic.
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