The wiliest dog on the park won the day for Harlequins
Leinster had a three-point advantage wiped away in the second half, at The Stoop, as Nick Easter (36 years young) heaved over and Quins winger Alesi Tikoirotuma snatched a Rob Kearney pass and raced in a crucial try.
Ian Madigan kicked six penalties for the visitors but the tries were the difference as Harlequins claimed their Champions Cup clash 24-18. The only consolation for Leinster was the losing bonus point.
There was little to raise the pulse in the opening 40 minutes and greasy handling conditions quelled attacking intentions from both backlines. Devin Toner made a bustling break in the early stages and Quins needed Mike Brown to put his body on the line to halt his charge. Eoin Reddan went off for a 10-minute Head Injury Assessment before returning to the fray.
By the time the scrum-half was back (after 11 minutes) the score was 3-3 as Ian Madigan and Nick Evans traded peanalties. The New Zealand out-half limped out of the contest soon after and, at the second time of asking, Tim Swiel, his replacement, put Quins ahead.
Jimmy Gopperth made a superb line-break midway through the half and, with the home side resorting to illegal means to stop the blue waves, Madigan levelled the game. Rob Kearney should have put Darragh Fanning away up the right wing but his pass was too low, and forward. Gopperth had one bite at the penalty cherry but his long-range effort fell short.
The most intriguing contest of the first half was the scrum battle between Mike Ross and Harlequins' loose-head Joe Marler. There has been a lot of chat in between scrums and Ross has won two penalties after losing a couple of early battles.
Madigan put Leinster ahead shortly after the break but Conor O'Shea's side steamed back and were rewarded when Nick Easter moonwalked his way to a crucial try. The Television Match Official needed only four replays to award the try.
Leinster pressed for a try that would have got them back in from but Fijian winger Aseli Tikoirotuma intercepted a Rob Kearney pass to Zane Kirchner and raced in an 80-metre try.
Ian Madigan chipped over a close-range penalty on 66 and 72 minutes to reduce arrears but Danny Care responded with a superb drop goal. Matt O'Connor's side will need a win, in bonus point style, to reassert control of the group, when the sides next meet at the Aviva Stadium on December 13.
