Fernandez-Castano travelled to El Saler immediately after competing in the Masters at Augusta, where he was disappointed to finish 20th after being in contention for much of the week.
"It's a shame that we have only one event in Spain this year and that's the reason why I am here," the 32-year-old said.
"Had it been a different tournament I would have taken a break, but it's my national Open. I know that Reale and the other sponsors, together with the (Spanish Golf) Federation have made a great effort to maintain it, so I'm happy to support it.
"I don't have the best of records in the Open de Espana and you get pretty tired after a major, but hopefully I can be up there if my short game is okay."
Fernandez-Castano has not finished better than 30th in his last five appearances in the event, but comes into it in good form in 2013.
Most notable was a third-place finish at the Arnold Palmer Invitational, where he shot 68 when playing with eventual winner Tiger Woods in the third round.
He also played with Woods in the third round at the Masters and admitted he was surprised to do so, given that the world number one had signed for an incorrect score the previous day.
Woods was given a two-shot penalty, rather than disqualification, after the tournament rules committee admitted they had reviewed his penalty drop on the 15th hole of the second round and initially deemed he had "proceeded appropriately."
"It was a strange day because we knew since early morning that he had a good chance of being disqualified," Fernandez-Castano added.
"It's a shame that a player should leave (a tournament) for making a mistake in good faith, but according to the rules, I thought I would play by myself.
"I was surprised that he wasn't disqualified. I didn't know the rule they applied (33-7), but it's there and I respect the decision, though I'm not fully convinced that it was applicable in this case.
"I was curious about the public's reception to Tiger, and the support he got was spectacular. On top of it, he birdied the first, and the uproar was deafening. I will never forget that day.
"Tiger is back at number one and seeing as I saw him play, he will be difficult to beat. Rory McIlroy is not at his best right now, but if he recovers there could be a nice duel for the first place.
"I wouldn't leave out other players like Adam Scott, though. After his boost of confidence at Augusta we could see him win more majors."
Fernandez-Castano was among the later starters today alongside compatriot Miguel Angel Jimenez, who was making his return to action after suffering a broken leg in a skiing accident in December last year.