Leeds secured their passage into the semi-finals of this year's Powergen Challenge Cup courtesy of a victory over Harlequins at Headingley Carnegie, which had looked in the balance for some time.

It will be their 12th last-four appearance in 14 years while the win will also serve as a boost to coach Tony Smith who missed the game due to illness.

Smith was in hospital suffering from gastroenteritis and handed the coaching reins over to assistant Brian McDermott.

McDermott, a former winner of the trophy with Bradford, will have been heartened to see his charges progress, but more so at the way in which they eventually blew away their opponents with a devastating second-half spell after looking in danger of losing a 12-0 lead.

Despite ideal conditions, both sides struggled to settle in the early exchanges and such was the lack of invention on show that neither side got on the scoreboard until 32 minutes.

The first half-hour was notable only for a string of errors by both sides, with Danny Williams spurning Leeds' best chance, while Chad Randall threw an aimless pass into the arms of the covering Lee Smith as Quins threatened to score.

Henry Paul, man of the match for Bradford in the 2000 final against Leeds, was the stand-out player and was unlucky not to post the first score himself after 25 minutes, narrowly failing to collect his own kick-and-chase.

But the Kiwi, now in his second spell in rugby league, was unable to do anything as Willie Poching, so often Leeds' saviour from the bench, collected Kevin Sinfield's pass to hurdle the cover defence and score to the left of the posts. Sinfield goaled to open up a six-point lead.

Poching's try was the catalyst for Williams, who would later complete his hat-trick, to make up for his earlier indiscretion and score in an identical position after excellent work from Rob Burrow, while Sinfield's goal handed Leeds a welcome but flattering 12-0 interval lead.

The tables were soon turned though and, by the 53rd minute, Harlequins were back in the game. First, Luke Dorn exploited a hole in the Leeds defence to race 30 metres to the line, before Michael Worrincy reduced the arrears further.

Dorn's try represented the first points Leeds had conceded in just over 200 minutes of rugby, although Worrincy's touchdown may well have hurt more.

With Williams seemingly dealing with Dorn's innocuous kick through, the winger then switched off allowing Nick Bradley-Qalilawa to creep in behind him and palm the ball back to the waiting Worrincy who gleefully crossed. Rob Purdham converted the latter and the score line was left at 12-10.

Any worries Leeds may have had were soon banished thanks to a tremendous burst of four tries in eight minutes, three of which involved Scott Donald.

The Australian winger scored two himself, collecting from Chev Walker and Shane Millard, while also supplying Danny McGuire with his 20th of the season. The other try of the quartet came from Williams, while all four raised question marks about Harlequins concentration levels at such a crucial time in the contest.

Pat Weisner and Tyrone Smith then posted a pair of consolation tries for Harlequins before Williams brought the curtain down on proceedings with his third try, a scrappy effort after Burrow's kick caused panic in the visitors' defence.