Fitzpatrick claimed his seventh DP World Tour title at Valderrama last year, but struggled to an opening three-over-par 74 on Thursday to lie eight shots off a five-way tie for the lead and four behind younger brother Alex.
"I just felt like I got Valderrama-ed today," Fitzpatrick said.
"I counted there to Billy (Foster, his caddie) on the last that I had six tee shots where I had to chip out sideways, played those in four over and a soft bogey on the last to shoot three over.
"Outside of those I played solid enough but it's just the way the golf course is; hit it in the wrong spots and you don't have a shot."
Fitzpatrick still rates Valderrama as a "great golf course" and believes patience will remain the key for players in contention, but added on Sky Sports: "It's a little bit different for me now.
"I'm a bit behind the eight ball so I need to shoot a low one tomorrow (Friday) to get back in contention. There will probably be a little bit less patience and little bit more aggressive playing to try and make up a score.
"It's not really a golf course you can push round; there are a couple of holes where you can be a bit more aggressive but on the whole you've got to plot your way around, but when you need birdies you've got to try and do something."
England's David Horsey held a share of the lead after an opening 66 which included seven birdies and a double bogey, with last year's runner-up Min Woo Lee, Jazz Janewattananond, Pep Angles and Soren Kjeldsen also five under.
"I played lovely," Horsey said. "If you're in the fairway there are a lot of chances out there.
"The weather was fairly benign this morning, soft conditions and no wind - it's a dream for pro golf. You see it all over the world, guys throwing darts at pins and it sticking.
"Valderrama kicked in on seven, my 16th hole, with the double bogey. It bit me a little bit but I would have taken that score at the start of the day."
Scotland's Robert MacIntyre, who won the Italian Open last month, was one shot off the lead alongside Spain's Adrian Otaegui.
Alex Fitzpatrick, who turned professional earlier this season, held an early share of the lead after three birdies in his first six holes, but eventually had to settle for a one-under-par 70.