South Sydney star Damien Cook working on his defence with UFC champ Robert Whittaker

Having secured his future at South Sydney this week, Damien Cook now focused on becoming the NRL’s best defensive No.9 with the help of one of Australia’s toughest fighters.

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Rabbitohs hooker Damien Cook is continuing his private training sessions with Australian UFC champ Robert Whittaker in a move that could see him become the greatest tackling No. 9 in the NRL.

While he may have secured his financial future this week, thanks to a $4.5 million extension at South Sydney, The Daily Telegraph can reveal Cook will continue looking to improve his game through one-on-one wrestling sessions at Whittaker's Smeaton Grange gym.

It has now been a year since the 27-year-old first struck up a friendship with the world's No.1 middleweight, who also doubles as a mad Souths fan.

This level of commitment is why Cook is worth so much money. (Brett Costello)

The Redfern rake has previously credited the grappling sessions for helping him to a breakout 2018 season that included representing NSW, Australia and now signing a rich five-year deal.

And while Cook isn't due back at Souths training until after Christmas, Whittaker confirmed the pair were still training at his Gracie Jiu Jitsu gym.

“I don't want the story to be that we're training together every day," Whittaker said. “Because that isn't the case. But Damien comes out every now and again.

"Obviously playing in the NRL, the rugby league side of things will always be his focus. But we just work on a few little things together.

"He's a great guy who works really hard."

Whittaker can certainly add to Cook’s weaponry. (Sam Ruttyn)

Despite being a livewire strike weapon in attack, it was Cook's perceived weaknesses in defence that saw him seek out Whittaker via trainer Alex Prates, who doubles as Souths wrestling coach.

According to the Fox Sports Lab, Cook made 957 tackles this past year, which was more than all but Roosters hooker Jake Friend (1148) and St George Illawarra opposite Cameron McInnes (980).

When it comes to tackle efficiency among the game's busiest defenders, meanwhile, Cook had a success rate of 90.8 per cent.

That put him behind the likes of Brisbane No. 9 Andrew McCullough (94.7), McInnes (92.9), Manly forward Jake Trbojevic (92.6), Melbourne skipper Cam Smith (92.1) and Friend (91.5).

Which is why the incumbent Kangaroo is continuing his grappling sessions with Whittaker.

“Wrestling can really help NRL players defensively," the UFC champ says. “It gives footballers a better understanding of how your body balances, how to apply your weight, grips to control you opponent's weight and balance … all of that can only help in the way you tackle.

“I think it really helps a rugby league player's defensive game.”

Cook is now Australia’s best hooker. (AAP Image/David Rowland)

Set to defend his middleweight belt at UFC 234 in Melbourne next February, Whittaker said he was learning plenty himself from Cook, who has taken over the Kangaroos hooking role following the rep retirement of Cameron Smith.

"Oh, he's a super athlete," says the fighter who faces American Kelvin Gastelum at Rod Laver Arena on February 10.

"And that's the biggest thing I've realised since networking with guys like the South Sydney players, I feel like I'm learning more from them than they are me."

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