The veteran openside, who has toured three times with Britain and Ireland's elite, was typically outstanding as he helped secure a tremendous away win against a side with one of the finest pedigrees in the European game.
"It's one of the biggest wins we've had as a region in seven years," said Williams, who is nearing 80 appearances in continental competition.
"It's not the main European competition but it's still a huge prize. When you look at the sides that were left in the Challenge Cup, Toulon have world-class players and we've got a huge amount of respect for Wasps with the players they've got.
"It is a relief, especially after the way we went out last year."
Williams doesn't dwell on last season's European exit, but mere mention of that incredible day at the Millennium Stadium is a reminder of the heartbreak he and his team-mates were forced to suffer.
Saturday's semi-final win came 12 months after an excruciating exit at the same stage of the Heineken Cup when Williams was off target in a first-ever penalty shootout as the Blues fell to defeat against Leicester.
This time, things ended differently against English opposition as Williams and co ensured history and disappointment didn't repeat themselves in the final four despite appalling weather conditions that prevented the Blues from employing their favoured expansive approach.
"If you look at our pack, we've got some big old units in there who like that kind of stuff in picking and going,' added Williams, who was also quick to praise a young scrum-half deputising for injured internationals Richie Rees and Gareth Cooper.
"You take your hat off to Darren Allinson who looked as though he'd been playing for ages. He was outstanding. He came into a pressurised environment and he bossed things really well.
"The support was also brilliant. We came off the bus and they were singing and to get that travelling support was brilliant for us.
"Hopefully we've got a bit of momentum now for the remainder of the season."
Momentum is certainly something the Blues have on their side. Victory at Wasps was their sixth successive win, with five of those coming away from home.
Another win over Munster next Saturday could see the Blues reach the Magners League Play-Offs, with a domestic semi-final then preceding a mouth-watering Amlin Challenge Cup Final clash with Toulon on French soil on May 23.
That showpiece event may still be three weeks away but Williams admits it's a tie that's been on his mind ever since the semi-final draw was made last month.
"Perhaps we shouldn't have, but we've been talking all week about the chance to play Toulon in Marseilles.
"It's like a once in a lifetime opportunity. There'll be a huge partisan crowd and a great atmosphere. It's the type of game that all too often in the past we've been sat watching to be honest.
"They've got quality all down their team sheet and they're flying in the French league.
"It'll probably be totally different conditions too. It'll probably be a hard ground, with the sun on their backs and just down the road from them but that's what we want and hopefully we'll rise to it."