The pre-event points leader was awarded pole for race one, after fog caused qualifying to be cancelled and grid positions were set by championship order.
He ran second into the closing stages behind fast-starting Red Bull Ferrari driver Felipe Fraga but, in an attempt to claim the lead at the final corner, the pair made contact and both retired after spinning.
The extra point Bortolotti would have earned for fastest lap went instead to Kelvin van der Linde, and he also was given a five-place grid penalty for race two.
Starting ninth, Bortolotti had moved up to fifth after a safety car restart and was attempting to pass van der Linde for fourth at Turn 4 when contact with the Abt Audi driver cut his left-rear tyre and put him out.
Van der Linde's brother, Schubert BMW driver Sheldon, was the main beneficiary of the race one incident to take his third victory of the season, then extended his points lead over Bortolotti in race two despite only finishing ninth.
Reflecting on the disappointing weekend, with Bortolotti now trailing by 21 points, Grasser told Autosport's German language sister title Motorsport-Total.com: "That was a weekend to forget. Of what we could have done, we simply didn't put anything into practice and take it with us.
"You forget weekends like that, you focus on the next 164 points to get. That will be the focus now."
While conceding that "it would certainly have been wiser to finish second and fifth", Grasser felt Kelvin van der Linde was at blame for the race two clash.
"In my opinion, Kelvin has to open up there, because they were side by side," he said.
"When someone is that deep into it, the other one has to open up. That wasn't exactly fair."
Unsurprisingly however, Kelvin van der Linde had a different view on the incident.
Speaking to Autosport after finishing fourth with 15kg in success ballast, he said: "Naturally I was very slow in the [Turn 3] left-hander and I knew he would at some point try to go right.
"I saw it once or twice in the mirror, him attempting it, but I was always wise to it.
"The final attempt I would say I was again wise to him trying, he was closer than ever and I think I heard him even selecting first gear to get a bit more torque off the corner.
"Straightaway I opened my exit, and then not expecting him to still go even further to the right, I went for the corner. I think he went over the kerb and still tried to make it work.
"He then overshot the corner, so I initially thought I could cut him back but then unfortunately he got a puncture, I think that side of it was a bit of bad luck for him.
"But in a normal racing scenario probably I would have undercut him again. We would have gone side-by-side into the next corner and maybe I would have had the inside again.
"It’s hard to judge. I didn’t expect him to go that extreme to the right because I was already far to the right.
"If he was clever, he would have stayed left, prepared the right and then he would have easily overtaken me."
Bortolotti was unavailable for comment.
Sheldon van der Linde took pole for race two and led the opening stages before being jumped in the pits by eventual winner Luca Stolz and then shuffled back at the restart.
Reflecting on the race with Motorsport-Total.com, the South African said "it was extremely surprising that Mirko didn't drive the race to the end again”.
"That helped me again, of course, because I think today we definitely lost a lot of places at the restart,” he added.
"That cost us a bit, but nevertheless I'm proud of the team for what they showed all weekend."
