Low clouds made play tough for the afternoon players, but there was no official stoppage. Play was eventually suspended for the night because of darkness.
Peter Baker carded a late bogey to post 67. He was joined in second at minus-5 by Notah Begay III.
Robert-Jan Derksen, the 2005 winner, shot 4-under-par 68 at Santo da Serra Golf Club. He shares fourth place with Daniel Vancsik, Mads Vibe-Hastrup and Martin Kaymer.
Santos, who has been a professional for a little over a year, started on the back nine with three straight pars. He traded a bogey for a birdie from the 13th.
The 24 year old caught fire late on his opening nine, rolling in a birdie putt on the par-3 17th and coming right back to birdie 18.
Santos kept rolling around the turn as he birdie one, two and three to make it five in a row. He moved to 6 under with a birdie on the par-4 fifth, then carded four straight pars to close his round.
'It's a good start, but I know I have a lot to do,' Santos admitted. 'It is hard to make the move from amateur to professional, but I have practiced and practiced very hard to help me with my ambitions. Today the difference was my putting.'
Baker jumped out of the gate with birdies on the first two holes. He notched back-to-back birdies for a second time from the fifth.
The three-time winner on the European Tour parred five straight around the turn before moving to minus-5 with a birdie on 12. Baker climbed to minus-6 with a birdie at 15, but gave that stroke back as he bogeyed 17.
'The time I had off with The Ryder Cup, watching it all and being so close to it, was definitely inspirational,' said Baker, who was a vice captain on the European Ryder Cup squad last year.
Begay, a four-time winner on the PGA TOUR, notched two birdies on the front nine. Around the turn, three more birdies got him to 5 under as he carded a bogey-free round.
Santiago Luna shot 3-under-par 69 and is tied for eighth. He was joined there by 2005 U.S. Amateur champion Edoardo Molinari, who has completed 16 holes. Sven Struver, Sam Little, Simon Nash, Oliver Wilson, Fredrik Andersson and Christian Cevaer share 10th at minus-2.