Slattery led by one overnight and produced a steady round of three birdies without dropping a shot to finish the third day at 14 under for the tournament, two ahead of Lorenzo Gagli of Italy and Brett Rumford of Australia.
Defending champion and No.1-ranked Luke Donald had a frustrating end to his round of 70 with three bogeys in the final five holes to finish 8 under going into the final day.
Donald bogeyed Nos. 16 and 18 – just as he did Friday – to stall his move up the leaderboard.
Slattery, who needs a good performance in Madrid to boost his chances of retaining his tour card for next season, birdied just one hole on the front nine but added two more at the 13th and 15th.
Gagli was far more erratic. The Italian eagled the par-5 No.4 but his four bogeys were accompanied by five birdies to finish 12 under. Rumford’s round of 68 included five birdies and a lone bogey at the par-4 13th.