In testing conditions at Lumine Golf Club in Tarragona, the Ayrshireman carded seven birdies as he opened with a four-under-par 67 in the six-round marathon.
Walker’s encouraging effort on the Lakes Course was sparked by three birdies from the second before adding further gains at the seventh, tenth, 13th and 18th.
The day’s work left the 24-year-old, who turned professional straight after the Walker Cup in September, sitting joint-eighth in the battle to finish in the top 25 and ties.
Daniel Young and Marc Warren were next best among the Scottish hopefuls in the 156-strong field with 69 and 70 respectively on a day when American John Catlin set the pace with a six-under-par 65.
Young, who also passed a second-stage test less then a week ago, signed for four birdies in his opening salvo on the Lakes Course to sit in a share of 33rd spot.
Warren, meanwhile, is in joint-50th aftrer a roller-coaster round on the Hills Course, making six birdies but also having an early double-bogey 7 as well as three bogeys.
Liam Johnston and Craig Howie both signed for level-par 72s on the Hills Course, with Ewen Ferguson carding the same score on the Lakes layout.
Johnston, who is facing this test after finishing 148th in the Race to Dubai, was two-under early on before dropping four shots around turn but recovered to make two birdies in last five holes.
Howie signed for three birdies and three bogeys while Ferguson mixed two birdies with two bogeys.
Catlin, a 29-year-old Californian, set the pace with a bogey-free effort on the Lakes Course, leading by one from a six-strong posse that includes Englishman Toby Tree.
Frenchman Gregory Havret, a former Scottish Open and Johnnie Walker champion, sits alongside Walker after opening his bid with five birdies.
But Welshman Jamie Donaldson, who holed the winning putt for Europe in the 2014 Ryder Cup at Gleneagles, had to settle for a one-over 73.
It was a disastrous start for Italian Matteo Manassero, the former BMW PGA champion sitting second last after a six-over 78.