The former champions missed out on a bonus point but a penalty try and Nicky Robinson's solo effort saw them edge out a spirited Dragons side for whom Dan Evans' claimed a solitary try.

Tom Prydie missed the target with a penalty attempt with just a minute gone and Evans followed suit with a failed drop goal but the Dragons did finally take the lead after 13 minutes.

Nicky Robinson hit back with his first penalty after 20 minutes - four minutes after Andrea Masi's chargedown of Evans' attempted clearance looked like spelling danger for the Dragons.

It was the Dragons who claimed the game's opening try on 25 minutes, though, as Evans picked up Rob Sidoli's flip pass off his toes 10 metres from the Wasps line.

Prydie's conversion had the hosts seven points clear but a second Robinson penalty closed the gap before Wasps were celebrating a try of their own.

The celebrations almost started after Evans was charged down for a second time, with Prydie then failing to tidy up under pressure from Jonathan Poff but Masi couldn't quite get downward pressure as the ball dropped from a height.

Wasps didn't have to wait long to claim their opening seven-pointer, though, as they took the resulting scrum past 90 degrees and then drove straight through the Dragons on their attacking ball. Referee Marius Mitrea had no hesitation in pointing to the posts when both Dragons flankers became detached from the setpiece to see the visitors take the lead for the first time.

Robinson's straightforward conversion made it 13-10 in Wasps' favour but Prydie ensured it was all square at the break when he slotted his second penalty with the final play of the half.

Prydie struck again with another penalty 12 minutes into the second 40 but there was bad news to come for the Dragons when Toby Faletau saw yellow for failing to roll away after his side had been warned repeatedly for breakdown offences.

Robinson took advantage of the extra man almost immediately as he collected his own grubber just to the left of the sticks for Wasps' second try.

The conversion had Wasps 20-16 to the good after 56 minutes but a long-range penalty from substitute Steffan Jones pulled the Dragons back to within a point with six minutes remaining.

The Dragons finished strongly but the Wasps' defence held firm to ensure they head into the last eight unbeaten.