Wales prop Tomas Francis has admitted he endured some "dark days" on his rapid journey to a huge World Cup stage that will await him at Twickenham this weekend.

Francis' rise through the ranks is well-chronicled, having played for Doncaster and London Scottish before enjoying a sparkling debut season with Aviva Premiership club Exeter last term.

The York-born tight-head, whose grandmother hails from Abercrave, wins his fourth cap on Saturday when Wales tackle fellow Pool A heavyweights England in a game that will shape both teams' quarter-final ambitions.

And while the seemingly ever-present smile tells all about how much he is enjoying living the dream, Francis admits his transition to Test rugby has sometimes proved challenging.

"International rugby is different from the Premiership and I've had to get used to that," he said.

"It was a shock to the system when I first joined the (Wales) squad and there were some dark days during the (July) training camp in Qatar.

"I didn't know if we were going to get through it but it's here now and that is in the bank.

"The rest of the boys got me through it. No-one wanted to drop out and you thrive in that environment. Everyone is in the same boat and you get through it together.

"I never dreamt a year ago that I would be playing in the World Cup against England at Twickenham - it hasn't sunk in yet.

"Things have happened so quickly I've not had time to stop and think. It's a game of rugby at the end of the day and you have to do the basics well, which is something I have always done and which has got me here.

"My conditioning and fitness have come on massively since I've been with Wales. My impact around the pitch has come on a lot and I have worked on my scrummaging."

Francis' scrum battle with livewire England loosehead Joe Marler will be a key mini-contest within the overall framework of Saturday's showdown.

And the 23-year-old has revealed the role played by 115 times-capped Wales front-row colleague Gethin Jenkins in preparing him to meet set-piece challenges at the highest level.

"In the Premiership, you can switch off in the scrum and you can get through it if you recover late," Francis added.

"If you're not there the whole time in a Test you will know, and it will show with a penalty against you.

"Gethin Jenkins has taken me under his wing a bit and it's been good to learn off him. He's told me not to be lazy, scrummage every scrum and he will look after me around the pitch.

"We've had a few referees in our scrummaging sessions, pointing out what the laws are, but it seems that every referee is different and you have to see what they want and change the picture for what they want to see.

"This World Cup has been a lot about the looseheads and their angle and them turning in. If I make sure I have a long bind and stay straight, I give Gethin a chance to attack his side and have a go at (England tightead) Dan Cole.

"The front-five is where they will try and take us on and where it will be won and lost.

"I played against Joe Marler once early last season when Exeter beat Harlequins. It went alright, but Quins didn't have the best scrummaging pack then.

"He has a great pack around him now and, like us, they have been together a long time. He's a strong boy and has a big reputation."