The Ospreys' Jonathan Thomas will have a late fitness test before their clash with Ulster © Getty Images

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Ospreys will have the opportunity to close the gap on leaders Leinster when they travel to Ravenhill to face Ulster on Friday night.

The Ospreys, who beat Ulster at home in round two, are ten points behind leaders Leinster, who face fourth-place Glasgow at Firhill on Saturday. The Welsh region overcame Aironi at home last weekend in Steve Tandy's first match in charge following the departures of Scott Johnson and Sean Holley and forwards coach Jonathan Humphreys is optimistic they can extend their two-game winning run.

Humphreys said: "It's good to go to Ulster off the back of two wins but we know that there's going to have be a real improvement from last week if we are going to make it three from three.

"Ravenhill is always a difficult place to go, even more so when they are in the kind of form they've shown during their European campaign this season. It's likely to be a full house, a passionate atmosphere, and a difficult evening for what is largely a young Ospreys team, with a sprinkling of experience.

"It's nights like these where you learn a lot about a team, about the character and spirit of young guys. It's going to be a great test."

Jonathan Thomas will have a late fitness test for the Ospreys while Ulster welcome back Pedrie Wannenburg, who packs down at No 8 in place of Robbie Diack, after a foot injury. Paul Marshall also returns to the starting XV with Ruan Pienaar moving to fly-half and Ian Humphreys dropping to the bench.

Leinster are on a 12-game winning run - with their only defeats this season coming against the Ospreys and Saturday's opponents Glasgow. The Warriors overturned Leinster in Dublin in round three and victory would increase their grip on a top-four slot.

After back-to-back wins over Treviso, third-place Munster face Cardiff Blues at Thomond Park. The future of Munster coach Tony McGahan remains in doubt with the Queenslander being linked with a role in Australia's coaching set-up under Robbie Deans.

Ronan O'Gara, Donnacha Ryan and Peter O'Mahony have returned to international duty for Ireland while the Blues are without New Zealanders Xavier Rush and Casey Laulala as well as their international contingent.

Elsewhere Edinburgh tackle Connacht in Galway on Saturday while bottom-side Aironi host the Dragons at the Stadio Luigi Zaffanella on Sunday with the Welsh Region looking to back-up their victory over Edinburgh last weekend.