Williams will be looking to play to his potential against the English and make an impression on watching Lions coach Sir Clive Woodward ahead of the tour to New Zealand.
"Martyn is one of the leaders, one of the senior guys," said Toulouse star Thomas. "I played in the same team as him when he won his first cap.
"He is one of those guys who has been here consistently throughout the tough times, and he's been one of the ones that has really dug in there.
"It's a great occasion to win your 50th cap, and he really deserves it. I hope the crowd on Saturday give him the reaction and support he has really earned."
Williams, who has made a quicker-than-expected recovery from a disc problem in his neck, will pack down alongside Dafydd Jones and Michael Owen as Wales look for their first victory over England in Six Nations history.
England's injury problems - no Jonny Wilkinson, Mike Tindall, Will Greenwood, Stuart Abbott, Richard Hill or Martin Corry - have increased expectation among Welsh supporters of an overdue home win.
However, Wales coach Mike Ruddock has been quick to inject a dose of reality.
"I don't think England will be vulnerable," he said.
"They beat South Africa in the autumn, we couldn't do that, they've got a great coaching set-up and they only just lost to Australia in their last game.
"They are a very powerful team with a lot of very good combinations and a lot of good balance about them.
"It's going to be difficult for us, but we are all up for it big time and looking forward to it."
The midfield battle between England's Jamie Noon and teenager Mathew Tait and the Welsh duo of Gavin Henson and Tom Shanklin promises to be a highlight.
Henson insists his long-standing groin problem will not restrict him in a game where much is expected of him.
"I have been on anti-inflammatories every day, I've been cut down to one training session a day, and it is feeling all right," he said.
"I've felt it for the last six or eight weeks, a gradual thing that got worse, but it was nice having the weekend off and it feels fine now.
"I don't mind being a marked man because hopefully by attracting players on to me it creates space for others. That's what normally happens, so it should make it easier."