Jamie Sutton, bronze medalist in 2014, was one of a handful of top extreme kayakers who didn't even make the final, along with two-time champion Joe Morley (Great Britain), 2014 Green Race Champion Isaac Levinson (United States) and 2009 Freestyle world champion Nick Troutman (Canada).
Serrasolses, 25, couldn't believe he'd held off Sutton's hard charge.
"I don't know what to say, if this is destiny or just that I am a really lucky guy. I have always been very lucky in my life and this was another lucky moment because Sam has been killing it throughout this whole event and he should have won, but I got luckier today and I can just be grateful for that," Serrasolses said.
Great Britain's Sandra Hyslop won the women's title from France's 2013-14 champion Nouria Newman, with Martina Wegman (Netherlands) third.
Despite Newman clocking the fastest time in the semifinals, she couldn't repeat that effort in the final, with Hyslop recording 1.08.82 to take her first title by 0.46secs.
Many of the top paddlers in the field are heading to New Zealand next month for the inaugural WhitewaterXL, which makes its debut on the new Vector Wero Whitewater Park from November 20-27 in Auckland.
Sutton and Newman are ambassadors for the event, alongside New Zealand's canoe slalom Olympians Mike Dawson and Luuka Jones, which will pit the world's best slalom and extreme paddlers against each other in a series of races.
Wegman is also confirmed to race, while 2008 Olympic champion Alexander Grimm (Germany) and rising Italian star Zeno Ivaldi also made the Sickline finals as preparation for the New Zealand event.