De La Riva carded six birdies and two bogeys to maintain the slender advantage he held overnight as he seeks a remarkable victory.
Ranked 610th in the world, the 29-year-old does not even have a European Tour card and has only once come through the qualifying school in nine attempts.
But he can draw confidence from finishing third at the Madrid Masters last October and has golf in his blood. His father was Spanish amateur champion seven times and his mother won the Spanish junior title on three occasions.
De La Riva’s nearest challengers are England’s former BMW PGA Championship winner Simon Khan and Italian teenager Matteo Manassero, who carded rounds of 66 and 68 respectively.
England’s David Lynn and South African Hennie Otto are another stroke back on 10 under, as is local favourite and tournament promoter Miguel Angel Jimenez, who has helped to keep the event afloat by dipping into his own pocket.
“I need to win to break even,” said Jimenez with a smile yesterday, although he may not be joking.
The 48-year-old is trying to become the oldest winner in European Tour history on the course where he is an honorary member and added a 69 today to previous rounds of 69 and 68.
The top Scot was Alastair Forsyth, who shot a 71 to put himself in a tie for 39th place on four under par.
That was two shots better off than Marc Warren, who was in a tie for 51st alongside, among others, his compatriots Gary Orr and Richie Ramsay.
A shot worse off than that group were George Murray, Stephen Gallacher and David Dysdale, who were in a bunch in joint 60th place.
Colin Montgomerie, Craig Lee, Peter Whiteford and Steven O’Hara all missed the halfway cut on Friday.