The Englishman, whose last and only European Tour win came seven years ago, birdied the first from six feet and despite handing the shot back at the fourth, he birdied the par five next to be one under for the day through nine holes.
That put Foster ten under - the mark Johan Edfors had set as the halfway lead.
But the Swede was not enjoying his third round as he searches for a first European Tour title since 2006.
Four bogeys in his first seven holes had taken him back to six under and into a tie for ninth.
Big-hitting Alvaro Quiros had taken over the pursuit of Foster, the Spaniard moving into contention at his home Open Championship with four birdies and an eagle across his opening 13 holes to sit nine under.
Paraguay's Fabrizio Zanotti was one under through 11 and into sole possession of third on eight under, one ahead of Wales Jamie Donaldson, Italian Francesco Molinari, France's Raphaël Jacquelin and home duo Carlos Del Moral and Sebi Garcia.