The Ospreys beat Harlequins 24-23 at the Liberty Stadium in Swansea on Sunday, with a last-gasp conversion from James Hook to clinch the reigning EDF Energy Cup champions first-round win.
Ospreys replacement Tommy Bowe scored a breakaway try three minutes into injury-time, and Hook converted in nerveless fashion from the touchline to win the game.
Hook earlier booted four penalties, giving him 14 points, while wing Shane Williams scored from the game's best move.
Quins scored second-half tries from centre De Wet Barry and hooker Tani Fuga, with Fijian fly-half Waisea Luveniyali added three penalties and two conversions.
Hook booted two penalties for a 6-0 lead after 25 minutes, with Quins only opening their account through a Luveniyali penalty on a first excursion into Ospreys' 22.
The Welsh were side were kept in check through a combination of their own over-eagerness and Quins' admirable commitment, but they conjured a memorable try as half-time approached.
Hook's defence-splitting pass sent lock Alun Wyn Jones galloping clear, before some slick passing saw Henson put Williams over in the corner.
It was a move that would have unlocked most defences, and Quins knew they had to keep things together as the interval approached.
But they trooped off one player down after referee Tim Wigglesworth had little option other than to sin-bin Quins' powerhouse Tongan wing Epi Taione.
Taione's illegal tackle on Hook saw Wigglesworth reach for a yellow card, and the Ospreys were close to taking charge at 14-3 ahead.
Quins though, dominated the early second-half flurries after Ospreys' former Ulster wing Bowe replaced Sonny Parker alongside Henson in midfield.
Wing Charlie Amesbury attacked the Ospreys from deep, and although Robshaw was denied a try from an ensuing ruck, Tosh Masson sent his centre partner Barry over for a well-crafted score.
And after 56 minutes Quins, Fuga touched down under a pile of bodies for their second try of the game.
Referee Wigglesworth required confirmation from video official Tony Rowlands before the try was awarded, before Luveniyali's conversion put Quins 17-14 in front.
Luveniyali then kicked a 45-metre penalty, which was quickly cancelled out by Hook's short-range strike, before an intriguing Ospreys tactical switch saw Williams move to scrum-half.
Ryan Jones was sin-binned for a technical infringement and handing Luveniyali an easy penalty chance that he gratefully accepted.
Jones and the Ospreys though, were saved by Bowe's dramatic late score that was created by full-back Lee Byrne's break and precision kick into space.
For the Ospreys:
Tries: S Williams, Bowe
Pens: Hook 4
Tries: Barry, Fuga
Cons: Luveniyali 2
Pens: Luveniyali 3
Ospreys: 15 Lee Byrne, 14 Jonny Vaughton, 13 Sonny Parker, 12 Gavin Henson, 11 Shane Williams, 10 James Hook, 9 Rhys Webb, 8 Ryan Jones (c), 7 Marty Holah, 6 Filo Tiatia, 5 Alun-Wyn Jones, 4 Ian Gough, 3 Adam Jones, 2 Huw Bennett, 1 Duncan Jones.
Replacements: 16 Richard Hibbard, 17 Andrew Millward, 18 Lyndon Bateman, 19 Steve Tandy, 20 Tom Smith, 21 Daniel Biggar, 22 Tommy Bowe.
Harlequins: 15 Tom Williams, 14 Epi Taione, 13 De Wet Barry, 12 Tosh Masson, 11 Charlie Amesbury, 10 Waisea Luveniyali, 9 Andy Gomarsall (c), 8 Phil Davies, 7 Neil Mcmillan, 6 Chris Robshaw, 5 George Robson, 4 James Percival, 3 Mark Lambert, 2 Tani Fuga, 1 Aston Croall.
Replacements: 16 Dan Frazier, 17 Ignacio Elosu, 18 Ollie Kohn, 19 Tom Guest, 20 Gareth Williams, 21 Seb Jewell, 22 Seb Stegmann.
Referee: Tim Wigglesworth (England)
Touch judges: David Rose (England), Bob Mullis (England)
Television match official: Tony Rowlands (Wales)