Richard McEvoy and Matthew Morris, a pair of Englishmen, were joined in third place after rounds of 7-under-par 65. Oskar Bergman, John Bickerton, Gary Emerson, Gregory Bourdy and Markus Brier were one stroke further back at minus-6. Brier aced the par-3 fourth.
Scahill got of to a flying start on the back nine at Santo da Serra Golf Club. He holed a sand-wedge second shot for eagle at the par-4 10th and came back with a birdie at the 11th.
The 35-year-old was not done either. Scahill rolled in a birdie putt at the par-4 12th and made it three in a row as he birdied the 13th to play the first four holes in 5 under par.
Scahill drained a birdie try on the 18th to make the turn at 6 under. He picked up birdies at the third and seventh on his way into the clubhouse to share the first round lead at 8 under.
'I had to stop myself from thinking about the course record after going to the turn in 30 and just get my mind back on today's round,' said Scahill, who owns the course record of 61. 'I made a great start. I was very happy with my performance in Portugal last week and I am ecstatic about today's.'
Sullivan, a Challenge Tour player, was steady to start his round. After four straight pars, the Welshman birdied the par-4 14th. Two holes later, he birdied the 16th to get to minus-2.
Around the turn, Sullivan rolled in back-to-back birdie putts from the first. He jumped to 6 under with an eagle on the par-5 third. The 28-year-old converted a birdie attempt on the sixth and grabbed a share of the first round lead with a birdie on the eighth.
'I think, as a Challenge Tour player, you have to prepare for and treat the dual ranking events as majors,' Sullivan said. 'And it worked out today. I got going, sank some nice putts and all of a sudden you've got a 64. I just stayed very patient all day because I started on the back nine and missed a 4-foot putt for birdie on the 11th, but I just told myself to keep smiling and enjoy it and it was a lovely day.'
Nicolas Colsaerts and Robert-Jan Derksen each posted a 5-under-par 67 to share 10th place. They were joined there by Benn Barham, Miguel Angel Martin, Andrew McLardy, Marco Ruiz, Garry Houston and Mikko Ilonen.
Andrew Coltart leads a group of 11 players one stroke further back at minus-4. Coltart recorded a rare double-eagle when he holed his second shot at the par-5 11th.