An opportunity like this has been a long time coming for Williams who celebrated his 31st birthday on Easter Monday.
Patience and perseverance have become his greatest allies since his first-team debut against Connacht in December 2009.
At different times, he has had to compete for the coveted Munster nine shirt behind three sitting Ireland scrum-halves: first Peter Stringer, then Tomás O’Leary and, for some time now, Conor Murray.
To avoid becoming discouraged when sitting for so long in the shadows, Williams has had to train his mind as hard as he does his body.
In his first season on a full contract, Williams played a central role inside Paul Warwick in Munster’s 15-6 win over the visiting Australians on a horribly wet evening in Thomond Park in November 2010, a game in which second row Ian Nagle also signalled his arrival.
Here were two players many thought would be the bedrock of the Munster team during this decade.
From Douglas in Cork, Duncan learnt the rudiments of the game at Cork Constitution in Temple Hill and, from there, attended Christian Brothers’ College on Sidney Hill, where he was part of a talented group that included Billy Holland, and one that swept all before them through the various age-grades: McCarthy Cup (U14), two Junior Cups (U15), Bowen Shield (U16) and a Senior Cup in 2003 when he and Holland were fifth year students.
At one stage he would have played with Tomás O’Leary, his senior by a couple of years, in CBC. They would cross paths in later years vying for the Munster No 9 shirt.
Coaches that Williams has worked under admit he can be very self-critical, a perfectionist in many ways but ferociously competitive.
When CBC lost to arch-rivals PBC in the first round of the 2004 Senior Cup, Williams was so annoyed with his performance he arrived home in a rage, flinging his cup jersey in the bin.
“He was so disgusted by his performance,” remembers his then CBC and Ireland Schools coach, Peter Melia.
“He set very high standards. What stands out is his skillset. He is very left-footed but, in terms of his passing, he was very natural off both hands from a very, very young age.
"When you meet him you wouldn’t identify that he’s extremely and ferociously competitive but on the rugby field he’s a different animal.”
He sits amongst a select group of schoolboys to be picked for two successive years for Ireland Schoolboys, winning a Grand Slam in his first year, touring Australia and during those two years played in august company alongside Johnny Sexton, Rob Kearney, Sean Cronin, Devin Toner, Andrew Trimble, and Darren Cave.
Throughout his career, Williams has had to contend with numerous injury setbacks.
In June 2006 he tore his cruciate in an U20 Rugby World Cup match, and needed to postpone the last 12 months of his Academy training at Munster.
At the time when rehabbing his knee in the Cork Con gym, he spoke about those tough days working on his own.
“I was in doing rehab five or six days a week. There were some very depressing mornings, like on a Friday and Saturday I was in there by myself, doing all the work. But these things make you a bit stronger, stand to you for the future.”
Soon after his return to full fitness, he fractured an eye socket while playing for Con in the AIL during the early part of the 2007/08 season, but he showed the perseverance to bounce back again.
Back then his burning ambition, he said, was to play for Munster.
“I’m a Cork man and I want to play my rugby with Munster, so hopefully that will come true,” Williams told the Irish Examiner at the time.
“He’s also tough and durable,” adds Melia. “He’s an excellent tackler and puts his body on the line. He’s a good guy to have to go to war with.”
This has been arguably his best season for Munster since signing his first professional forms and, earlier this year, he was awarded a two-year extension to his current deal.
He commented wryly that “there was probably a collective groan around the province when it was announced”, perhaps an indication that he may feel unloved by certain sections of the Munster support base.
Interestingly, he tweeted recently that it “affects more than just the person” after posting words by Ian Keatley on his page in which his teammate described the effect the booing in Thomond Park last season has had on his family.
Playing Saracens at a packed Lansdowne Road in front a huge television audience will represent a high point in Williams’ career.
He’ll face another challenge to his position next season when James Hart arrives from Racing 92 but he is well used to competition. He’ll understand his place in the pecking order.
The great triathlete of the 1980s Paula Newby-Fraser once said in reference to using rivals to improve herself: “When you compete against someone of equal or greater ability, it can lift your energy and mental concentration to new heights.
"The key is to see your rivalry as your guiding light, as a tool you can use to find the potential within yourself.”
Williams knows this. It takes patience and perseverance.