Ospreys 18-20 Leinster
Dan Biggar missed a dramatic injury-time penalty as Guinness Pro12 leaders Leinster held on to win at the Liberty Stadium.
That was after replacement fly-half Ross Byrne kicked a final-minute drop-goal to snatch back a lead the Ospreys had held since the hour mark.
Both sides scored two tries with Leinster coming from hooker Sean Cronin and flanker Dan Leavy while Ospreys scores came from Sam Davies and Justin Tipuric.
Biggar made his 200th Ospreys appearance but was not up against Lions rival Johnny Sexton, with Joe Carbery starting at fly-half as one of eight Leinster changes.
But despite the shake-up Leinster were still able to field an all-Ireland international front row of Jack McGrath, Cronin and Tadhg Furlong.
Ospreys clung on to a slender 11-10 lead at half-time helped by an explosive start which saw them score a try after only five minutes after Biggar's opening penalty shot had rebounded off the right upright.
Josh Matavesi and Olly Cracknell combined well to help Ospreys to break in to the 22 before Davies made it into the right corner.
Biggar missed the conversion but was on target on nine minutes from in front of the posts after Leinster conceded a penalty.
At that stage Leinster were not in the game but the tide turned dramatically when Cronin was put away down the right sprinted impressively round Biggar and under the posts for Nacewa to convert.
Five minutes from the break Leinster stole the lead with a Nacewa penalty after Ospreys failed to release at a ruck after good work by Josh van der Flier.
But in the final minute of the half Biggar wrestled back the lead with a 45-metre penalty shot.
Leinster regained the lead seven minutes in to the second half when a break for the line by Cronin was finished off by Leavy, who claimed his side's second try for Nacewa to convert.
But on the hour mark Davies, who moved to fly-half with Biggar going centre, found a fine touch and from that position Ospreys won the line-out with Rhys Webb going close to scoring before Justin Tipuric squeezed over. Davies converted to give Ospreys a slender lead.
But in the final minute Byrne truck an impressive drop goal that looked to have sealed win.
However, three minutes into injury time Byrne turned villain when he was penalised for a high tackle on Dan Evans.
But Leinster celebrated as Biggar's penalty kick sailed wide of the target.