Corser, riding for Alstare Suzuki, won race one with ease and looked to be on course to double up in the second after starting from pole position.
But Yamaha Italia rider Haga discovered a new turn of pace in the second event and charged through the field after starting 17th to pip Corser to the chequered flag and claim one of the most unlikely wins in the series' history.
Corser accepted second best for a result which stretches his championship lead to a near-insurmountable 94 points over Winston Honda's Chris Vermeulen, who finished eighth and third. The reigning champion James Toseland emphasised his recent return to form with second, in race one but a disappointing showing in the day's other race virtually ended any hope of keeping the title.
The Ducati Xerox rider started from second position but quickly slid down the order and had to make do with eighth place, which leaves him a massive 147 points down on Corser.
Carl Fogarty's Foggy Petronas team had a weekend to forget, with Steve Martin and Garry McCoy failing to pick up a single point from the two races.
McCoy's day ended on a painful note when he crashed out of race one, badly gashing an elbow which kept him out of race two. The Australian clashed with Jose Luis Cardosa on lap five and was hit by Mauro Sanchini, cutting his elbow, hurting his ankle and suffering concussion. The injuries left McCoy furious with Cardosa.
"I had seen Fonsi Nieto and Cardoso having a dice and Cardoso was running wide quite a bit, which allowed me to catch him up fairly quickly," McCoy explained. "He went into turn 15 very wide, but I just braked and kept my line. I didn't see him because he was so wide but at the last minute I caught this blue colour in the corner of my eye and then there was a big hit in the side of the bike.
"I thought I was going to save it, but Sanchini couldn't avoid hitting me from behind. I am not very happy about it. When you run wide it's normal to look before coming back in."Reuse content