American teenager Taylor Fritz upset second-seeded Steve Johnson 7-6 (5), 7-6 (7) in the second round of the Memphis Open in Memphis, Tenn., on Thursday night.
Fritz, a wild card, survived 12 aces and became the first 18-year-old American to make an ATP World Tour quarterfinal in five years.
"I played the big points well," Fritz said. "He had a lot more chances than me. He didn't do anything wrong, but I played just about as well as I could play on the big points. I think that was the difference."
Johnson, an American ranked No. 29 and eight years Fritz's senior, applied heavy pressure on the world No. 145's service games in the opening set. Fritz saved seven break points after dropping serve in the first game before breaking back and securing the tie-break on his fourth set point.
"I'm happy for the victory," Nadal said. "The start was tough, as Juan is a good player and the big humidity here in Argentina made it difficult. I am happy to be through and hope to be ready for tomorrow."
Monaco of Argentina had not played a tournament since undergoing right wrist surgery in August.
Nadal will face Paolo Lorenzi of Italy, who defeated Diego Schwartzman 4-6, 6-3, 6-1 of Argentina.
Martin Klizan saved five match points to beat Roberto Bautista Agut 6-7 (5), 7-6 (6), 6-0 in the World Tennis Tournament semifinals in Rotterdam, the Netherlands.
Klizan will face qualifier Nicolas Mahut, who defeated eighth-seeded Viktor Troicki 6-2, 6-1.