London - Harlequins took control of Pool 2 with a 24-18 victory over three-time champions Leinster in London on Sunday.
That put them clear on top with 12 points, three ahead of the Irish, who took a losing bonus point, while Wasps are another two points further back.
At the Twickenham Stoop, Quins and Leinster were embroiled in an arm-wrestle for 50 minutes before veteran Nick Easter scored the hosts' first try.
By then Quins had lost New Zealander fly-half Nick Evans to injury but not before he responded to Ian Madigan's opening penalty.
Evans's replacement Tim Swiel twice put the hosts in front from the tee, only for Madigan to level up and the sides went into the break at 9-9.
Flex their muscles
Madigan's fourth penalty gave the three-time champions the lead but Easter turned, twisted and stretched out his arms over his head as he was brought to ground to just inch the ball onto the line on 54 minutes and give the English the lead, which was extended to 16-12 by Swiel's conversion.
Quins had their tails up but a great kick chase from Darragh Fanning took Leinster deep down into home territory.
They spun the ball out to the right but Asaeli Tikoirotuma picked off Rob Kearney's blind pass and ran back 90 yards to add a second score, although Swiel missed with the extras.
That was the cue for Leinster to flex their muscles as Quins paid for a series of errors, with two more quick-fire Madigan penalties making it a three-point game.
But scrum-half Danny Care took the Irish by surprise five minutes from time by knocking over a drop goal with his left foot to seal the win.
In Pool 4, record four-time champions Toulouse took over top spot from Glasgow by beating the Scottish side 19-11 in the south of France.
Glasgow could have snatched a losing bonus point but Duncan Weir missed a late conversion from Nikola Matawalu's try.
Maxime Medard had already crossed the whitewash for Toulouse, who lead the Scots by three points with Bath another four behind, while Sebastien Bezy added 14 points with the boot.