The prince, who is the vice patron of the Welsh Rugby Union, was in Cardiff on Thursday to watch his beloved Wales take another giant step towards the quarterfinals with a 23-13 win over Fiji in Cardiff.
William, with a red scarf wrapped around his neck, was at Twickenham last Saturday as Wales produced a historic performance to seal a 28-25 win over old enemy England.
And this latest win will give all Wales supporters hopes of reaching the last eight with just Australia in London to come next Saturday.
Wales, bloodied and bruised after its enthralling win over host nation England , has had its squad ravaged by injury.
It had lost Leigh Halfpenny, Jonathan Davies and Rhys Webb before the tournament had even started, while wing Hallam Amos and centers Scott Williams and Cory Allen were also missing.
Liam Williams, the full-back, suffered concussion against England so was also absent.
While Wales began the day as favorite, the ghosts of 2007 World Cup in France still lingered.
On that occasion, Fiji shocked the world of rugby by defeating Wales 38-34 to reach its first quarterfinal since 1987.
Wales gained revenge four years later, crushing the Pacific Islanders 66-0 on the way to reaching the semifinals where it was defeated 9-8 by France.
But the most recent meeting, which took place in November 2014, was a far tighter affair with Wales prevailing 17-13.
This contest turned out to be rather similar with Fiji giving Wales a real fright in its own backyard.
While Wales led 17-6 at the break courtesy of tries by Gareth Davies and Scott Baldwin, the visitors rallied in the second half.
Vereniki Goneva rounded off a wonderful flowing Fijian move to ruffle a few nerves although Wales held out in the end.