out from the line before putting the ball on the boot and running through to plant it down inches inside the dead-ball line.
When Todd Carney converted to make it 14-6 it was game on.
The Roosters needed a strong start after the break and they did just that to level the scored inside the first three minutes, albeit in controversial circumstances.
With Manly deep on the attack, a Chris Bailey knock on was scooped up by Kenny-Dowall outpaced both Robertson and Lyon on a 90 metres race to the line.
The Sea Eagles complained that Matai was taken out by Roosters winger Perrett in the lead and it was referred to video ref Sean Hampstead, who gave the incident the all-clear and awarded the try.
Hampstead then infuriated the Manly players by ruling Lyon struck Kenny-Dowall as he dived for the line, giving the home side a chance at an eight-point try.
Carney slotted both shots at goal to bring the scores level.
Manly appeared to be losing their cool and soon after Glenn Stewart was penalised and placed on report for a late hit on Braith Anasta which is sure to come under scrutiny by the Match Review Panel.
An astonishing momentum shift had now come over the game and it was no surprise when Carney put the Tricolours in front when he stepped past a tired Matt Cross to score under the posts.
Carney converted his own try to make it 20 points in just 10 minutes for the Roosters to lead 20-14 with still more than half an hour to go.
It was end-to-end action now with a real finals-like intensity and it's the Roosters who will go into September with all the momentum as Perrett and then Kenny-Dowall crossed in quick succession to make the result safe.
Sydney Roosters 30
Tries: Kenny-Dowall 3, Carney, Perrett
Goals: Carney 5
Manly Sea Eagles 14
Tries: Robertson, Matai
Goals: Lyon 3