Cramp has denied David Reynolds and Luke Youlden a slice of Bathurst immortality, striking at the most inopportune moment to deny the pair back-to-back titles at Mount Panorama.
The Erebus Motorsport team arrived at the 1000-kilometre classic as champions.
They were fastest in qualifying and won pole.
And they were leading inside the last hour of the race, only for Reynolds to suffer the debilitating ailment as he aimed for home.
"I'm so devastated. We had the fastest car all day," he said.
"I've never had that happen before in anything I've ever driven ... I can't believe it."
After reporting the issue and seeing second-placed Craig Lowndes eating into his lap times, Reynolds' pit team sprung into action.
Youlden, who was completing a warm down, threw back on his race suit and helmet in case he was needed.
On the track, Lowndes jumped Reynolds just before the window opened for his final pit stop of the day.
Insisting on continuing, Reynolds took on hydrating drinks and Berocca in the hope of battling on.
Unfortunately, he spun his wheels while hoisted - bringing a drive-through penalty which killed his race hopes.
He pitted once more after serving the punishment, handing over racing duties to his co-driver, who finished 13th.
Reynolds needed to be helped from his Commodore and sat slumped in the garage with his head in his hands.
"I just can't believe it," he repeated, as if in a trance.
"My leg was cramping big time. When Lowndes passed me ... I couldn't tell if I was on the brake or on the throttle.
"I wasn't mentally there. I was lost.
"My body gave up after that. I'm so disappointed for everybody involved."
Team principal Betty Klimenko said it was a bittersweet day.
"Its sweet to see Craig go out with a Bathurst win but its bitter because we led all weekend and in 90 per cent of the race," she told AAP.
"David's kicking himself. He knows what he did was wrong.
"A champion's red mist came over him. He thought he could get home because he wanted so badly to win again."
In a shattering 10-minute spell for Erebus Motorsport, young gun Anton de Pasquale also came unstuck.
The 23-year-old, who started from third place, hit the wall to buckle a wheel, ending his day 18 laps from home.
Klimenko said that was "more upsetting than Dave's incident".
"They're both gutted but they'll get over it. They're young and will go for it again," she said.