Kiefer lost to South African qualifier Neville Godwin, 6-3, 3-6, 6-4, while Youzhny was downed by Swede Magnus Larsson, 4-6, 6-3, 6-4.
No. 4 seed Davide Sanguinetti of Italy came from behind to avoid the same fate of his ranked peers, advancing with a 6-7 (1-7), 6-1, 6-2 victory over Spaniard Juan Balcells.
In unseeded play, American Cecil Mamiit of the United States recorded a straight-sets victory over France's Anthony Dupuis, 6-0, 6-3, and hometown favorite Kenneth Carlsen of Denmark also won in straight sets, defeating Australian Wayne Arthurs, 6-4, 6-4.
Carlsen, a Copenhagen native, faces Sanguinetti in the second round.
Swim Week Miami Beach 2014 [PHOTOS]