Home hero Jon Rahm opened up a one-shot lead as the Open de España was set for a dramatic finish at Centro Nacional de Golf.
Irishman Paul Dunne started the day with a one-shot lead but he played the front nine in level par, with Rahm picking up three shots in his first ten holes to get to 18 under and leapfrog him.
Madrid native Nacho Elvira was also at 17 under, one shot clear of clubhouse leader George Coetzee and two ahead of England's Andy Sullivan.
World Number Four Rahm has seven National Championship victories as an amateur at various age levels but this would be his first professional victory on home soil.
It would also make him the third fastest player to three European Tour wins in terms of events played after last season's two Rolex Series triumphs in Ireland and Dubai.
Rahm played a beautiful chip at the first to set up a birdie and when he hit a huge drive on the second to set up another gain, he was in a share of the lead.
Elvira also birdied the par five first and there was very quickly a three-way tie at 17 under.
Dunne started his round with three pars while Rahm did well to save par on the fourth after going a long way left off the tee, and he needed to save par again after another ragged tee-shot on the fifth.
He soon found himself in the solo lead, however, as Elvira and Dunne both missed the green and failed to get up and down on the fourth.
The 23 year old was not there for long as Elvira got the biggest cheer of the day so far on the fifth, holing a downhill left-to-righter from 35 feet for eagle to leapfrog his countryman.
Dunne made a birdie after a nice chip on the par five to get back to 17 under.
Rahm had missed a good birdie chance on the sixth but he made no such mistake on the seventh, leaving himself a flick in and holing from four feet for a birdie.
He was once again the solo leader when a poor approach to the sixth led to an Elvira bogey but Rahm bogeyed the ninth and there was once again a three-way tie.
Rahm did not look likely to take advantage of the par five tenth after sending his second right and his third over the green but he chipped in and led on his own.
Coetzee equalled the lowest round of the week with a 63.
The Tshwane Open champion birdied the first, second, fifth, sixth and eighth to turn in 31 and then eagled the tenth, adding further birdies on the 13th and last.
"It was nice," he said. "I actually told my caddie before the round, this is actually the first time I've looked forward to playing this week because I haven't been hitting it great, so it was nice to get a good hour of warm-up coming into today and it seemed to pay off.
"My driving was much better. I was hitting the wrong fairways the first couple of days and today I kept it on the golf course, so it was nice to keep the ball in play and have a lot more chances at birdies.
"My golf has been kind of good leading up to this week, it is just funny how after five weeks you have a lot of rust to work off, so looking forward to next week now."
Sullivan continued his remarkable comeback this week to remain in contention.
The three-time European Tour winner started the week with a 75 but then signed for rounds of 63-65 and was two under for the day after ten holes on Sunday.
He birdied the first and while he gave the shot back on the third, he birdied the seventh and then chipped in at the eighth.
Julien Guerrier was in the clubhouse at 14 under after a 64, with Jorge Campillo, Henric Sturehed and Marc Warren alongside him playing the back nine.
Alejandro Cañizares, Rak-hyun Cho, Sébastien Gros, Yusaku Miyazato, Adrian Otaegui and amateur Victor Pastor were in the clubhouse at 13 under.
