Does tenacity decrease Warwicks ult damage?
Tenacity: The duration of stuns, slows,
taunts, fears, silences, blinds
and immobilizes are reduced by 35%.
Suppress is not affected by tenacity.
Source: http://leagueoflegends.wikia.com/wiki/Mercury%27s_Treads
Tenacity does not reduce Suppression or Airborne, Quicksilver Sash/Mercurial Scimitar actives WILL prevent the rest of the damage, but that's not tenacity.
See this question for more details. How does Tenacity work?