Ospreys host Harlequins at Brewery Field this Sunday with both teams still in with a chance of qualifying.
The home side must win and hope other results go their way; Harlequins need just two points to secure qualification.
Rhys Webb will skipper an Ospreys team that has been hit by injuries and international call-ups.
They do include Wales internationals Jonathan Thomas and five-time capped Kiwi Campbell Johnstone.
Rory Clegg returns at fly-half and Ross Chilsholm starts at fullback after coming back from injury last week.
Luke Wallace will lead the side as they aim for a home semi-final.
Ospreys: 15 Jamie Murphy, 14 Tom Grabham, 13 Jonathan Spratt, 12 Ben John, 11 Dafydd Howells, 10 Matthew Morgan, 9 Rhys Webb (c), 8 Morgan Allen, 7 Arthur Ellis, 6 Jonathan Thomas, 5 Sam Williams, 4 Lloyd Peers, 3 Campbell Johnstone, 2 Scott Baldwin, 1 Marc Thomas.
Replacements: 16 Matthew Dwyer, 17 Duncan Jones, 18 Dmitri Arhip, 19 Ian Gough, 20 George Stowers, 21 Dan Baker, 22 Tom Habberfield, 23 Kah Fotuali'i.
Harlequins: 15 Ross Chisholm, 14 Seb Stegmann, 13 Matt Hopper, 12 Ben Botica, 11 Sam Smith, 10 Rory Clegg, 9 Karl Dickson, 8 Tom Guest, 7 Luke Wallace (c), 6 Joe Trayfoot, 5 Charlie Matthews, 4 Sam Tworney, 3 Will Collier, 2 Dave Ward, 1 Mark Lambert.
Replacements: 16 Henry Allen, 17 Darryl Marfo, 18 James Johnston, 19 Charlie Walker, 20 Maurie Fa'asavalu, 21 Jordan Burns, 22 Louis Grimoldby, 23 Ollie Lindsay-Hague.
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The Lions are ready to get their Australia tour underway. They arrive in Perth on Monday.
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