The Italian thus took also the overall ahead of Manuel Monni, who finished a distant second in the second moto, while Stephen Sword clinched the final sport on the overall podium as he tied on points with Cairoli (eleventh in race two).
Rattray was another high profile victim, finishing just 17th, but still kept the title lead - by just three points over Cairoli - with Guarneri now up to third and Searle holding fourth.
"I could not expect this victory after my injury last week. I hit my shoulder on a pole in a silly crash and I have torn a ligament," said Guarneri. "The doctor said that maybe it was not possible to race here. I tried on Saturday and did OK but I had a lot of pain. I had an injection for Saturday and also Sunday and the shoulder, luckily, was not so bad. The conditions were terrible however and so many riders stopped, but I had a good start and rode within myself. I am so happy with this win because I have had some bad luck with injuries at the start of a few seasons. I just want to run with the leaders at each grand prix and look as high as I can in the championship."
The first round of the FIM Veterans World Cup saw Dutchman Toine van Dijk take the overall victory. A disappointing tenth in heat one, Van Dijk went out for revenge in the second moto, leaving 2007 World Cup winner Dave Thorpe behind in second.
Second on the overall podium was Briton Scott Eastwood, who was second in heat one and sixth in heat two.
2006 World Cup winner Thierry Godfroid made a stunning comeback to racing after being injured himself at
Donington Park last year. Despite several months off the bike, the Belgian was sixth and fourth for third overall.
Poleman Peter Iven took fourth, while first moto winner and sole CCM rider Greg Hanson was fifth, after his second moto was compromised by a collision with another rider.
Thorpe was ninth after a difficult opening moto; he was second behind Iven when the latter crashed and Thorpe could not avoid him.
The FIM MX1 Motocross World Championship continues next weekend - April 27 - with round three at Agueda in Portugal.
