Leading by three shots at the start of the day's play, Fernandez-Castano first witnessed unheralded Filipino Juvic Pagunsan draw level him at the top of the leaderboard on 14-under, before seeing his tee shot at the 18th hole sail into the water.
However, the Spaniard recovered with a brilliant putt to save par, forcing a sudden-death play-off, which was twice suspended by rain after both players had hit two shots each.
With no let-up in the torrential rain, tournament organisers were forced to postpone the rest of the play-off to Monday morning, and despite letting his lead slip, Fernandez-Castano remains upbeat about his chances.
"It has been another long day waiting around for three hours, but tomorrow [Monday] is a new day and anything can happen," Castano-Fernandez said. "We are both in a good position on the fairway."
The Spaniard admitted he had a difficult day, first with the intense heat, and then the pressure as the tournament reached its final stages.
"It was a tough day out there - one of the toughest I have ever had. The heat was brutal and my game was not good.
"I was feeling the pressure because it has been a long time without a victory and I want this one desperately.
"I felt the pressure at the end. My game was just not there, the putter wasn't working and I made it tough for myself but at the very end, to make that par on the last made my day.
"Those three pars to finish - I know this doesn't sound great talking about three pars - but considering the position I was in, the way I was playing and the way my mind was, it was a big challenge and I managed to hang on."
And despite the relief of having forced a play-off, Castano-Fernandez is aware the pressure on him has now increased, considering he threw away a good chance to end a three-year winless streak.
"You never relax because I haven't won on Tour since 2008 and I really want to get the win.
"There was a sense of relief and it is now a shoot-out between the two of us."
Both golfers will return to the course at 7:30am local time on Monday to decide the winner of the 2011 Barclays Singapore Open.