England number eight Billy Vunipola says he was unable to walk following his ankle injury against Argentina, but wants to "build a legacy" after being declared fit for Saturday's World Cup quarter-final against Australia.

Vunipola was forced off at half-time during the 39-10 Pool C hammering of the Pumas on 5 October.

But he feels the injury may have been a "blessing in disguise" as Eddie Jones' side prepare to face the Wallabies.

"It's not a bad thing," he said.

"It's probably given me chance to recharge the batteries a little bit."

Vunipola added on the Rugby Union Weekly podcast: "When it happened, my first thoughts were to get to half-time and see where I was at.

"But when I got off half-time, I couldn't walk! So I made the decision [to come off] with one of the physios and with Eddie."

Vunipola has been heavily utilised by Jones in recent months, starting all four of England's warm-up games as well as the three World Cup meetings with Tonga, the USA and Argentina.

"I've played a lot of rugby recently, I've had a good run [with injury], and it just happens," he said.

"It's probably been a good little blessing in disguise to have these past few weeks off my feet and it's probably worked out in my favour - and hopefully in the team's favour as well."

'Knockout games are different'

Back in 2015, Vunipola was injured during the pivotal pool meeting with Wales, denying him the chance to rescue England's faltering campaign, which ended with a chastening early exit.

So like many of the current squad, Saturday's game in Oita will be his first World Cup knockout match.

"The feeling around knockout games different," he explained.

"There is more pressure and more expectation. But from my point of view, it is where you want to be.

"I remember clocking in [for pre-season], and I was dreading it. In the back of my head, every time we were doing hard training was 'this is where I want to be' [before a World Cup quarter-final].

"We have a big opportunity to grasp. That's kind of how you build your legacy, either by being successful or by being unsuccessful."

'My best performances are alongside Mako'

Vunipola's return to fitness coincides with a return to the XV for older brother Mako, who hasn't started for England since the Six Nations because of a hamstring injury.

And while Billy rejects the notion England are overly reliant on the two siblings, he admits that he often raises his performance levels when Mako lines up alongside him.

"It's unfair to say me and my brother hold the key to the team being successful - we just try to add as much as we can," he said.

"But yeah, I would probably say some of my best performances have been with my brother next to me. It is no coincidence and it gives me an easy feeling.

"For example when I bought my house, he organised everything. That's how I play rugby. He deals with all the admin, and I do all the fun stuff."