Fourth-seeded Nishikori struggled at times but managed to overcome former world No. 1 Juan Carlos Ferrero of Spain 7-5, 3-6, 6-2, despite surrendering five double faults.
No. 6 seed Wawrinka, of Sweden, dispatched Argentina's Horacio Zeballos 6-3, 6-4 in a straight-set victory on the clay courts at Buenos Aires Lawn Tennis Club.
Wawrinka earned the win in part by saving 6-of-7 break chances by his Argentinean opponent.
Victor Hanescu of Romania outslugged Spaniard Pablo Andujar 6-3, 4-6, 7-6 (7-4) Monday and Argentina's Carlos Berlocq rallied for a 6-7 (5-7), 6-0, 6-0 blitz of Spain's Pere Riba.