David Gilford, Gary Evans, Bradley Dredge and Mikko Ilonen all shot 70 to tie for second second slot. Rain halted play for more than two hours, allowing only 66 players in the 156-man field to complete their round.
A 24 year-old who turned pro last year and commutes from Raleigh, N.C. to play the European Tour, Petterson had seven birdies and one bogey Thursday.
A mix of narrow fairways, tight, bumpy green, intermittent rain and wind caused problems for nearly everyone. Italy's Costantino Rocca hitting three straight approaches out of bounds to make a 10 at the 12th but recovered for a 75.
Sweden's Klas Ericsson had a 13 at the par-4 14th and shot 91, Portugal's Stephane Ferreira had a 90 and Daniel Silva or Portugal was 17-over with three to play.
Full-field scores from the Algarve Open de Portugal