The iconic Willunga Hill will be taken out of Saturday's Tour Down Under stage because of the local bushfire threat and forecast 40C-plus temperatures.
The Santos Tour's Queen stage will start from suburban Brighton an hour earlier than scheduled, at 10.10am.
Originally ending with three laps of the Willunga climb and a summit finish, the 176km fourth stage will be shortened to 131km and finish in the Willunga township at the base of the climb.
The final fifth stage on Sunday, a circuit race at Stirling in the Adelaide Hills, is scheduled to go ahead as planned.
The decision was taken after race director Stuart O'Grady and other organisers met on Friday afternoon with local emergency services, riders and race judges.
There is an extreme fire danger rating on Saturday for the Mt Lofty Ranges, with the temperature set to hit 43C.
"While the removal of the famous Willunga Hill climb is disappointing news for cycling fans, ultimately rider and spectator safety is always the No.1 priority for our event," O'Grady said in a statement.
There had been speculation earlier on Friday that the whole Willunga stage might be cancelled, a first for the Tour that started in 1999.
The last stage of the 2014 Herald Sun Tour in Victoria was called off because of a bushfire threat.
After Sam Welsford won stage three on Friday, the Australian said he hoped Willunga would go ahead as planned - despite the climb not suiting his sprinting prowess.
"Everyone knows Willunga is such an iconic part of this race and everyone loves racing up there in the crowds," he said.
"But obviously there's a safety part of it as well."
Welsford secured his seventh Tour stage win, taking out a high-speed finish in the Adelaide Hills town of Nairne.
His new team Ineos Grenadiers led him out perfectly at the end of the 140.6km stage, and the 2024 Olympic track gold medallist was too strong for his rival fast men.
There was confusion immediately after the stage finish as to whether fellow Australian Jay Vine retained his healthy overall lead, a day after he and UAE Team Emirates colleague Jhonatan Narvaez dominated stage two and took a stranglehold on the general classification.
Initially, Narvaez was listed on the race coverage as the new overall leader ahead of Vine, but it was later confirmed that the Australian was still in the ochre leader's jersey.
There was a crash in the last 400m of the stage, but under race rules because it happened inside the last 3km, Vine's lead was unaffected despite being caught behind the tumble.
Vine said the last 50km of the stage was "really stressful".
"But I have a really good team around me, they were able to help me," he said.
"I was able to not crash in the final, but got help up at the end there."
Vine and Narvaez broke away on the decisive Corkscrew Rd climb near the end of stage two, with the Australian winning the stage by nearly a minute.
Given the significant advantage that those two riders have over the rest of the field, losing the Willunga climbs will not be seen as a massive impact on the overall results.
Vine is the latest Australian to win the Tour, claiming the title in 2023, and Ecuador's Narvaez is the defending champion.
There was some excitement near the end of Friday's stage when the remnants of the day's three-rider breakaway were holding off the peloton.
But the race came together inside the last kilometre and Welsford powered to a convincing win.
