Fernandez-Castano produced his lowest round of the tournament at Royal Park in Turin with an eight-under-par 64 to finish on 24 under and beat South African Garth Mulroy (67) by two shots.
Ryder Cup players Martin Kaymer (67), rookie Nicolas Colsaerts (67) and Francesco Molinari (65) were in similarly good form as they played their final competitive rounds before the Sept. 28-30 matchplay event against the United States.
Kaymer, the 2010 US PGA Championship winner, ended up 18 under overall for a share of fifth place, a timely return to form for the German whose best previous finish all season of seventh had come in April.
It was also the former world number one's highest placing since his HSBC Champions victory last November, a loss of form which has seen his ranking drop to 32.
"The last two weeks were a very nice feeling for me. I was thinking about whether I should play or practise more but it was a good decision I played in Holland and here," Kaymer told the European Tour website.
"After the PGA Championship I was thinking about taking six weeks off to prepare for one tournament, the Ryder Cup.
"I played really well this week. Last week was ok," he said of his tied-21st finish at the Dutch Open.
"I've worked so hard on my long game that I didn't spend that much time on my short game. So I've got to work on my game around and on the green."
Belgium's Colsaerts matched Kaymer's efforts to register his 10th top-10 finish in an outstanding season, while Italian Molinari recovered from his four-over-par third round with an eight-birdie 65 to delight the final-day crowds.
"It helps being able to go through the week in a very cool, calm way, without thinking about qualifying for The Ryder Cup," said Colsaerts.
"My goal was to play these two events and to keep going, keep playing and keep the good dynamic I've had all year. I wish I could've done a little better here, but it's fulfilled the goal of continuing to play well before going to Medinah.
"I have a lot of good vibes," added the world number 35, the first Belgian to qualify for the biennial competition.
The good vibes were clearly felt by European Ryder Cup captain Jose Maria Olazabal, who broke 70 for the first time all week with a closing 65.