The pair have been instrumental in firing the Welsh region into the European Cup quarter-finals but their Ospreys’ allegiances will be firmly forgotten in what promises to be a ferocious battle of the Celts.

And British Lion Williams is looking forward to his match-up with Walker - one of 13 Ospreys who will start the game - in what he is calling “a battle of wits’’.

"I think it will be great, I'm looking forward to it,’’ said Williams, scorer of 35 tries in 51 Wales appearances.

"Nikki is a great player and has had a great couple of seasons for the Ospreys since he joined from the Borders. He is a deadly finisher, a physical lad and is quick, one of the quickest in the Ospreys squad. He has got a lot to offer.

"Having trained alongside him we both know each other's strength and weaknesses so it will be a battle of wits this weekend. I am hopefully not going to be that easy to read and I am sure he will try and be the same.

"We both have different qualities so it will be an interesting battle. He has deserved his place in the Scotland side and he has been pushing for a long time.

"The move has helped him because, no disrespect to the Borders, since he has joined the Ospreys he has become a better player and with the Ospreys doing so well he has become more recognised. He is getting picked for Scotland, is first choice and deserves it.

"It does help when you are playing in a successful side, but he deserves his place anyway.’’

Though 30-year-old Williams has been a regular for Wales teams over the last few years he admits nothing is certain under the new national coaching team of Warren Gatland and Shaun Edwards, who have made three changes for the Scotland clash despite the 26-19 opening win at Twickenham.

"I need to be on top of my game to keep my place because boys are pushing for places. Nobody has that guarantee and that is good because it means no-one is becoming complacent.

"Your spot is never safe and you should never think that it is. Warren and the coaching staff have said if he does not think you are right for the game then he is not going to pick you which is very interesting for the lads.’’

On the threat of Scotland, who are licking their wounds after home defeat against France last Sunday, Williams said: “We have to respect Scotland because they are a resilient side with a good set-piece and can keep the ball moving.

“They will be disappointed with the score against France which I thought was quite flattering to France in the end. We can't take our foot off the pedal because Scotland will punish you.”