Harlequins' delightful December double over Stade Francais will count for nothing if they don't qualify for their first Heineken Cup quarter-final for 11 years.

That's the view of Tom Williams, who has been one of Quins' unsung heroes in recent weeks.

Williams admits the wins over the French giants will live long in the memory of Quins players as well as their fans.

But he insists the memory will be somewhat soured if they fail to win one of their last two group games to make the knock-out stages.

Williams said: "The game in Paris was the most exciting game I've ever been involved in.

"You don't often get a chance to play in front of 75,000 people, and I may never get that chance again.

"You've got to seize that opportunity, and to win over there, in the Stade de France against Stade Francais, is one of the highlights of my rugby career.

"Scoring a try was great, but it didn't even enter my mind after the game. The fact we had won was all I cared about.

"But we are still only six points clear, and who's to say that Stade won't win their next two games?

"And if they earn bonus points and we lost our next two games, it would all be in vain.

"We want to win both remaining games if possible to try and get a home quarter-final."

The first of those games sees Quins travel to Belfast on Saturday to take on Ulster at Ravenhill.

And although Quins thrashed the Irishmen 42-21 at the Twickenham Stoop back in October, Williams warned it won't be so easy this time round.

He said: "If you look at Ulster's recent results, they beat Munster away, and it wasn't as if it was close.

"They are a completely different outfit from the team we played in mid-October.

"They have made massive strides in defence and attack and they've got some high quality players such as Andrew Trimble, who I will be opposing this weekend, and you can't discount them at all.

"In fact, only Leicester have gone over there and won in recent years.

"But we are confident we can qualify."

However, Williams is all too aware that Quins have not won since those back-to-back victories over Stade Francais.

They have lost to Northampton and Wasps and drawn with Leicester, and a frozen pitch at the Stoop put paid to them putting things right against struggling Worcester last Saturday.

Williams added: "That's a very English way of looking at it, but you can't hide from the fact that we haven't backed up those performances on the pitch since those two games.

"We've been working hard in training, and it's not through lack of effort, that's for sure.

"I think it's just a couple of technical issues, and having won those games by the skin of our teeth, we've been icing over some of the cracks that have opened over the last couple of weeks."

* ANY match ticket purchased for the Worcester game will be valid for the rearranged fixture when a new date is set. However, refunds cannot be made until the rearranged fixture is set.