The durable American, a former two-time world champion who not been stopped in 14 years, represented a significant challenge for the Briton.
Haye, 29, notably avoided the kind of pre-fight trash talk which he had unleashed on previous opponent Nikolai Valuev, preferring to do his talking in the ring.
The Londoner remained supremely confident in advance of his head-to-head with a man who had gone 36 rounds over three fights against Evander Holyfield a decade earlier.
Haye said: "I don't believe that, on my day, anyone in the world can take the punishment I'm willing to dish out for 12 rounds.
"They may be able to take three or four rounds but sooner or later it will sicken them."
That confidence proved to be well-founded as Haye mounted a brutal defence of his title, knocking his opponent to the canvas four times by the time his trainer Miguel Diaz threw in the towel with 59 seconds of the ninth round remaining.
It left Ruiz, known as "the Quietman", contemplating his future – he did not fight again – and the victor a showdown with either Vitali or Wladimir Klitschko, who between them held the WBC, IBF and WBO belts at the time.
Haye said afterwards: "If I hit the Klitschkos with the same shots I was hitting John Ruiz, both of them would go over."
He got his wish the following summer when, after a routine defence against compatriot Audley Harrison, he met Wladimir Klitschko at the Imtech Arena in Hamburg.
The Ukrainian prevailed courtesy of a unanimous points decision and former world cruiserweight champion Haye announced his retirement three months later before reconsidering his decision.
He entered the ring on five more occasions but eventually hung up his gloves at the age of 37 in June 2018 after successive defeats by Tony Bellew.