Fernandez-Castaño had all six of his birdies on the way in, and the BMW Italian Open winner was delighted that he was able to stay patient after nine consecutive pars on the front nine.
“The back nine had nothing to do with the front nine,” he said. “I played very well on front nine, believe it or not. I didn't hole any putts, but I stayed patient, and all of the putts seemed to drop on the back nine.
“It was worth the wait, that's for sure.”
Warren is making his first appearance at the event since it moved to the Middle East in 2009 and his day began with a "mad rush" to find his waterproofs after stepping outside his hotel and seeing heavy rain – 20 millimetres of it in two hours - but it soon cleared up and he had perfect conditions for his round.
"There was no wind to speak of and the greens were pure. I felt as if I hit a lot of good golf," the former World Cup winner said.
His last victory was in 2007 and he blew a great chance at his home Aberdeen Asset Management Scottish Open in July, dropping four shots in the last four holes to lose by one just like Adam Scott at The Open Championship the following week.
"Winning is the most fun you can have at a golf tournament and it's something I want to do again soon," he added.
The tournament is limited to the top 60 money winners on The European Tour this year, but only 56 teed off. Thomas Björn and Retief Goosen were out injured, Ross Fisher pulled out earlier in the week, and this morning Ireland's Shane Lowry withdrew after going down with a virus that even put him on a drip in hospital for a while.
Sergio Garcia, returning after eye surgery to correct an astigmatism following The Ryder Cup, managed a 73 and Ian Poulter was on the same one over mark after mixing four birdies with five bogeys in his first 12 holes.
Joint fourth with 67s were 2009 winner Lee Westwood, Padraig Harrington, Martin Kaymer, Richie Ramsay, Louis Oosthuizen and Fredrik Andersson Hed.