Petrie had braced himself for pace-setters Aberdeenshire to seal the glory by beating Grange after his own side had been demolished in double-quick time by Watsonians.
But the Dons imploded in spectacular fashion at Raeburn Place, which means the whole shooting match will be decided in the Granite City on Saturday.
An almost disbelieving Petrie said: “It was a very bizarre Saturday. Our performance is extremely hard to explain. We usually back ourselves to post a winnable total, even on a tricky track – and that is why I opted to bat after winning the toss.
“I certainly wouldn’t blame our defeat on the fact we took first knock. I would credit ’Sonians for the quality of their new-ball bowling and there is no doubt they thoroughly deserved their win.
“Our preparations were the ones that had served us well in most of our other matches in the campaign, and I saw no sign of nerves or apprehension in the warm-up.
“We just kept losing wickets at regular intervals, which can happen to any team at any time. It was just unfortunate that it happened to us at such a crucial time.
“We will, however, put it behind us and move forward quickly and concentrate on this coming weekend because all is not lost!”
For Petrie and Co to keep the title they must stave off the challenge of Stoneywood-Dyce at People’s Park, while also banking on Falkland to pull off a shock win over Aberdeenshire at Mannofield.
Arbroath’s nine-wicket hammering had left Shire knowing a victory over Grange would settle the issue a week early.
And after skittling the home side for 145, they appeared to be hot favourites – only to capitulate for 115 all out.
On the green and damp track their pursuit had suffered an immediate setback when Chris Venske was clean bowled by Gordon Goudie in the opening over.
Then three wickets in quick succession set Shire further back on their heels, leaving them in a jittery position at 54 for four.
Nick Farrar came into the attack and instantly made an impression by removing Harsha Cooray, who edged to stand-in wicketkeeper Gregor Maiden. That left Aberdeenshire on a shaky 77 for five, with the outcome finely balanced.
And even worse was to come just seven runs later when Maiden played the major role in the run-out of Callum Martin. And Maiden was quickly back in the spotlight, combining with Farrar again to send Kenny Reid packing.
Farrar’s amazing stint wasn’t finished yet. In the 32nd over, he found his way past Tyler Buchan’s defences and he bagged another before the weather intervened to finish with five for 26.
Earlier, Dons skipper Buchan had demolished the Grange upper order, reducing them to five for three.
He struck with the first ball of the contest to get rid of Ryan Flannigan, then trapped former Scotland star and home captain Neil McCallum lbw in his next over. Farrar was his third victim as Buchan gleefully pouched a smart return catch.
Though Grange halted the immediate rot, Cooray snapped up three scalps himself before Buchan completed his five-for when shattering the stumps of veteran Maiden.