The six-time WorldSBK champion was unable to back-up his Race 1 podium after crashing out of the Superpole Race on lap one.
After losing out from pole, Rea attempted to regain P2 off Andrea Locatelli, but as he went to the inside of his future teammate Rea lost the front-end of his Kawasaki.
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Alex Lowes also crashed out whilst running directly behind Rea, which led to both riders remounting their machines before returning to pit lane.
“I made errors in both races, so it’s been a very disappointing day,” said Rea. “Starting with the Superpole Race, I felt that if I had a chance to win another race before the end of the season, it was there in that race.
“I was potentially too aggressive in the first laps. I felt ‘Loka’ was quite conservative in Turn 5 and I pounced but, as I was getting closer to the corner, that gap was closing.
“I put myself more at risk than him. I ran in there a bit hot on the inside and took a bit of kerb. Obviously, it’s very easy to do; my teammate followed in as well.
“As stupid a mistake as it was for me, I’m sorry for him because line of sight dragged him in there. It was the end of my race.”
A mistake that resulted in Rea being much further down the field for Race 2, the 36-year-old then collided with Xavi Vierge on lap one which led to a Long Lap penalty.
Rea’s error also resulted in Scott Redding being caught up in the accident, while Vierge’s race ended in the gravel.
Rea said: “That had a knock-on on Race 2, starting from way back in P10. I was in a position in Turn 3 where everything was closing in.
“I arrived there quite fast. Jumped on both brakes and could do nothing to avoid Xavi Vierge so the first thing I did when I pulled my helmet off was go and find him in Parc Ferme and apologise.
“I’m so sorry to destroy his race and that of Scott Redding’s. I haven’t seen Scott yet, but it was a mistake on my behalf.
“I took my Long Lap Penalty and tried not to give up, tried to keep my lap times consistent and bring my bike home to take some points home for the Championship.
“The big picture is we’re still racing for third in the Championship. That’ll go down to Jerez.”