The Ospreys lost ground on the top two in the Guinness PRO12 with a surprise 13-5 defeat to Treviso in Italy.
Steve Tandy's side were comfortable 64-10 winners in the corresponding fixture in Swansea but fortunes were dramatically reversed at the Stadio Monigo on Saturday night.
The hosts were on the front foot from the beginning, dominating possession, and took the lead midway through the first half when Alberto Sgarbi powered over the line. Tito Tebali, against his former club, added the extra points with the conversion as he made amends for an earlier penalty miss.
The Ospreys' performance was littered by uncharacteristic mistakes but they finally got going as half-time approached when Sam Davies ran down the line and off-loaded to Dan Evans before Keelan Giles ran in his 13th try of the season. Davies missed the conversion as the hosts led 7-5 at the break.
A Tebaldi penalty extended Treviso's lead in the second half while the other points came from the boot of Ian McKinley.
The defeat was only a second loss in the last 10 PRO12 games for the Ospreys and they remain in third place in the table. Their failure to pick up a bonus point means they are now six points behind leaders Leinster and four adrift of second-placed Munster.