The Reds hadn't beaten the Waratahs since 2013 and, cruising at 19-7 it looked like they suddenly remembered that as the visitors defied a weaker scrum, mounting penalty count and some poor handling to peg back the lead.

A try to Jack Maddocks through a gaping hole put the Waratahs up by four points early in the second half before Harry Wilson scored for the hosts and James O'Connor stepped up with two late penalties to win it.

The win came with prop Taniela Tupou sin-binned for 10 minutes late in the second half, the Wallabies enforcer penalised for twice knocking down the kicker.

"That was very physical, that was state of origin stuff," 22-year-old Reds captain Liam Wright said.

"We wanted to slap Nella around a bit but we just said 'what a story it would be; last 10 minutes of a brutal game, guys are tired and busted, if we can hold them up (despite being one man down) and come back to win the cup'."

The Waratahs had four debutants but it was an old-fashioned battle as both sides embraced the rivalry that had been flamed by constant banter in the lead-up.

"I'm massively impressed with the young boys coming through; the Reds have made some strides and we've got a crop coming through ... hopefully in the next 10 years there are going to be ding-dongs (battles)," Waratahs coach Rob Penney said.

The Reds crushed the Waratahs' scrum but were leaky in defence, both sides happy with the new law interpretations they said encouraged quicker, cleaner play and the ability for opportunistic defences to turn the ball over.

The novel 50-22 and 22-50 kicking rules also came into play, allowing the Reds to turn the momentum late in the second half.

Penney lamented a lack of discipline and some poor handling errors as the penalty count finished 18-9 in the Reds' favour.

"We showed some progress but 36 opportunities to turn the ball over, you're not going to be on the right end too many times," the coach said.

Wright and Fraser McReight combined well in their first start together in the Reds backrow, while No.8 Wilson continued his strong pre-shutdown form.

Wright scored first and thought he had a second when he ran around the ruck to plant a loose ball in front of a sleeping Waratahs defence.

It was deemed offside though, as was a Tupou effort moments earlier as the Reds threatened to run away with the clash.

Tate McDermott darted in from a quick tap, while Filipo Daugunu crossed in the corner and Harry Johnson-Holmes burrowed over for the Waratahs' first-half try.

Australian Associated Press