Swede Alex Noren, chasing his third victory of the season, led the way on 64 at the pro-am event that is being played on three courses -- St Andrews, Carnoustie and Kingsbarns.
Pieters birdied the par-four seventh at Carnoustie after hitting a monster drive on to the green at the 401-yard hole.
"It's great to be back. Everybody is on my side here," the big-hitting Belgian told reporters.
"But I'm still very tired mentally," added Pieters who was top scorer with four points from five matches at last week's Ryder Cup in Minnesota won 17-11 by the United States.
Spaniard Cabrera-Bello began his Dunhill campaign with four birdies and two bogeys at Carnoustie.
England's Ross Fisher was in second place on 65, one ahead of compatriot Matt Ford and Swede Joakim Lagergren.
(Writing by Tony Jimenez; Editing by Pritha Sarkar)
