Thiem will face Uruguay's Pablo Cuevas, a 6-4, 7-5 winner over Portugal's Joao Sousa.
Fourth-seeded Diego Schwartzman of Argentina beat Slovenia's Aljaz Bedene 6-4, 2-6, 6-2 to set up a match against Spain's Albert Ramos-Vinolas. Ramos-Vinolas beat compatriot David Ferrer 3-6, 7-6 (9), 6-3.
Ferrer promised to retire in May at the Madrid Open. The participation in the Buenos Aires tournament is one of the last of the 36-year-old's career.
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