Just a week after his emotional Millennium Stadium send-off when he retired from Test rugby, former Wales star Williams returns to action with the Ospreys against Aviva Premiership champions Saracens (kick-off 6pm).
The key Heineken Cup clash comes at the midway point of a tightly-contested Pool Five, which also features French heavyweights Biarritz and offers only one guaranteed quarter-final place.
The Ospreys, despite boasting a star-studded squad, have never progressed beyond the last-eight phase in Europe, making successive knockout-stage exits to Saracens, Munster and Biarritz.
"As a professional player, you really want to win trophies," Williams said.
"In international rugby it is the World Cup and the Six Nations, and in the club game it is the Heineken Cup. It's the Champions League of rugby.
"I would love to be involved in a successful Ospreys side in the competition. We have not asserted ourselves in the latter stages when we have got there, and it is what we need to do.
"Saracens were better on the day and taught us a lesson (in 2008). The Heineken Cup at that stage is very, very tough.
"We are playing a good side away from home, but it is an away game for Saracens, too, as it's at Wembley.
"We know we can play some good rugby and win some big games. It's not revenge or anything as we didn't deserve to win in 2008. It's about turning up on the day and giving a good account of ourselves."
Saracens' Namibian World Cup captain Jacques Burger, who lines up in a side that sees South African hooker Schalk Brits back after serving a three-week suspension, believes the group will continue to take no prisoners.
"You have to win your home games and get something away from home, and even finishing second is probably not going to be enough to qualify as one of the best runners-up," Burger said.
"We just have to bring a real edge to our performances in the next two games.
"We let ourselves down last season and we weren't mature enough to handle the demands of this competition.
"There are no more excuses for this team now. We cannot think about achieving something in the Heineken Cup next year or the season after - it's all about now."
Loan signing Peter Stringer, recruited from Munster earlier this week, is poised for his Saracens debut off the bench, with Brits being joined on the comeback trail by centre Brad Barritt and fly-half Charlie Hodgson.
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