Williams, who played in the second Lions Test against the All Blacks in 2005 after scoring five tries in the mid-week fixture against Manawatu, was among 14 Welsh internationals to appear for the Ospreys as they fielded two completely different XVs in either half.
A last-minute penalty try sealed a 30-17 win for the visitors as the Ospreys finished their first pre-season fixture with defeat despite tries from Rhys Webb and Andy Lloyd.
Will Hurrell crossed for the game's first try after 22 minutes, with former Newcastle Falcons fly-half Toby Flood converting to move the Tigers 10 points clear.
The England U-18 winger was involved three times in a flowing move that began five metres inside his side's own half. Jordan Crane's lengthy run put the Ospreys on the back foot before fellow England Saxons international Richard Blaze supplied the scoring pass. Hurrell stepped Lee Byrne to dive over underneath the posts and give Flood the easiest of conversion attempts.
The Ospreys hit back just two minutes later, however, as scrum-half Webb showed an impressive turn of pace to snipe his way over from eight metres out. The 19-year-old spotted a gap in the Tigers' ruck defence to register the Ospreys' opening try of the new season.
Leicester were next on the scoresheet with the game 33 minutes old thanks to a fine individual effort from full back Matt Smith. The former Tigers Academy player, who spent last season on loan at National One side Nottingham, beat three lacklustre tackles after fielding a clearance kick just inside his own half.
Again Flood converted, although this time from a far more difficult angle just a metre in from the left-touchline.
Dan Biggar and Flood then exchanged penalties either side of half-time before the Guinness Premiership finalists lost Crane to the sin bin for deliberately killing the ball after 56 minutes.
The Ospreys made the most of the numerical advantage as Lloyd crossed for his side's second try after Flood failed to hang on to a high pass behind his own line. Lloyd had the simplest of tasks to dive on the lose ball and close the gap to five points, while James Hook's straightforward conversion reduced the deficit even further, leaving the hosts trailing by just three points at 17-20 with 19 minutes left to play.
That was as close as the Ospreys would come to victory, though, as a further penalty from Flood and the converted penalty try sealed the win for Leicester.
Ospreys (first-half): Lee Byrne, Kristian Phillips, David Bishop, Andrew Bishop, Nikki Walker, Dan Biggar, Rhys Webb; Paul James, Ed Shirvington, Cai Griffiths, Ian Gough, Ian Evans, Jonathan Thomas, Marty Holah, Ryan Jones
Ospreys (second-half): Gareth Owen, Shane Williams, Tommy Bowe, Ashley Beck, Leigh Bevan, James Hook, Rhodri Wells; Duncan Jones, Marc Breeze, Craig Mitchell, Andy Lloyd, Alun Wyn Jones, Tom Smith (captain), Steve Tandy, Filo Tiatia
Used replacements: Sonny Parker, Ryan Bevington
Leicester Tigers: Matt Smith,Will Hurrell, Dan Hipkiss, Aaron Mauger, Tom Varndell, Toby Flood, Harry Ellis; Marcos Ayerza, Mefin Davies, Julian White, Richard Blaze, Ben Kay, Tom Croft, Ben Woods, Jordan Crane
Replacements: George Chuter, Santiago Bonorino, Dan Cole, Brett Deacon, Dan Hemingway, Ben Herring, Ben Youngs, Tom Youngs, Jack Cobden