The Spanish Fernandez-Castano, whose only tour victory came last year at the KLM Dutch Open, shot a 3-under 69 on Saturday. He is at minus-5 and is the only player in the field to post three rounds under par.
Peter O'Malley and Paul Casey both carded rounds of 3-under 69 and are tied with Mahal Pearce, who fired 4-under 68 in the third round, at 4-under-par 212.
Paul Lawrie, the 1999 British Open champion who was alone in second place after the second round, struggled to a 1-over 73 in round three. He is knotted in sixth place with Markus Brier, who shot a 2-under 70 on Saturday, at minus-3.
The second round was completed on Saturday as lightning storms caused the suspension of play on Friday. Stenson missed a 10-foot birdie putt on his final hole Saturday morning, but still took a one-shot lead into the third round.
Saturday's third round featured the same blustery winds that wreaked havoc on scoring Friday. The temperatures dipped quite a bit as well, which led to only 10 scores under par of the 73 players who made the cut.
Stenson extended his margin with a birdie at the very first hole, but dropped a shot at the third, his second bogey at the hole during the tournament. He bogeyed the sixth hole, which dropped him out of the lead thanks to a strong run by O'Malley.
Fernandez-Castano surged up the leaderboard thanks to four birdies in his first 11 holes. That got him to 6 under par and put him into a tie for first with O'Mally after the Australian birdied No. 14.
Stenson steadied himself with a birdie at the par-5 ninth to get back to 5 under par. Fernandez-Castano was alone in the lead at minus-6, and himself played solid golf on the back nine. He parred holes 12-17, but a bogey at the par-five closing hole dropped him into a tie atop the leaderboard with Stenson.
The Swede reclaimed sole possession of the lead with a birdie at 16. He parred his final two holes and is now one round away from claiming his second victory of the season on the European Tour. Stenson captured the Qatar Masters and a win on Sunday would be his fourth.
Fernandez-Castano birdied the second, sixth, ninth and 13th holes, but his untimely bogey at the closing hole dropped him out of a share for the 54-hole lead.
Eight-time Order of Merit winner Colin Montgomerie shot a 1-under 71 and is tied for eighth place. Jose-Filipe Lima only managed a 1-over 73 and joined Montgomerie at minus-2.
Graeme Storm, Simon Dyson and Jose Manuel Lara all posted identical rounds of even-par 72 on Saturday. The trio is knotted in 10th place at 1-under-par 215.
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