The Spaniard did not drop a shot in the fourth round, while his South African counterpart will rue holes six and 17, where he picked up bogeys.
Fernandez-Castano completed the fourth round eight under par to finish 24 under for the tournament.
"It felt like matchplay out there and I'm delighted to have come out on top," he said. "It feels very special to win this tournament again.
BOXING: Irish middleweight Matthew Macklin is back on track for a world title bid after a first-round knockout victory over former world champion Joachim Alcine in Las Vegas.
Fighting on the undercard of Sergio Martinez and Julio Cesar Chavez Jr, the 'Tipperary Tornado' twice put the Canadian on the canvas and the referee stopped the bout after just two minutes 38 seconds.
In the main clash, Argentina's Martinez survived a torrid final round to defeat Chavez Jr. and claim the WBC middleweight title.
A series of Chavez left hands knocked Martinez down, but the challenger hung on for victory.
RUGBY UNION: Australia avoided an upset after a second-half revival allowed them to escape with a 23-19 win over a spirited Argentina at Skilled Park on Saturday night.
Stunned by a feisty Pumas outfit playing their first match in Australia since the 2003 World Cup, the Wallabies looked set for a shock defeat after two Argentinian tries had them 19-6 down midway through the second period.
However, the quickfire double only served to spark the home side into action, with tries from Pat McCabe and man-of-the-match Digby Ioane sparing the Wallabies from embarrassment.
In the other Rugby Championship game, New Zealand beat South Africa 21-11.
NFL: NY Giants 41 Tampa Bay 34, Carolina 35 New Orleans 27, Arizona 20 New England 18, Indianapolis 23 Minnesota 20, Philadelphia 24 Baltimore 23, Buffalo 35 Kansas City 17, Cincinnati 34 Cleveland 27, Houston 27 Jacksonville 7, Miami 35 Oakland 13, Seattle 27 Dallas 7, St Louis 31 Washington 28, San Diego 38 Tennessee 10, Pittsburgh 27 NY Jets 10, San Francisco 27, Detroit 19.