Horn, the top seed for the men's singles draw, triumphed 15-7, 15-4 over South Korean opponent Moon Gyun Kim to top Group A in the Costa Rican capital.

The American's team-mate Charlie Pratt, the third seed, also impressed in beating Venezuela's Luis Felipe Zea 15-6, 15-13 today.

Mexico's Daniel de la Rosa and Rodrigo Montoya, the respective second and fourth seeds, could mount the biggest challenge to the United States duo.

De La Rosa thrashed Ramon De Leon of Dominican Republic 15-6, 15-1 today.

Montoya was equally comfortable in beating Fernando Kurzbard of Argentina 15-1, 15-10.

A rest day will take place tomorrow before knock-out rounds begin on Wednesday (June 16).

Mexico's three-time defending champion and women's top seed Longoria beat Jin Seok of South Korea 15-6, 15-4.

Her team-mate Samantha Salas won 15-3, 15-3 over Carla Muñoz of Chile.

American second seed Rhonda Rajsich also defeated Naomi Wakimoto of Japan 15-2, 15-3.

The biggest near-shock today came when Maria Renee Rodriguez of Guatemala was taken to three games by Aisling Hickey of Ireland before winning 11-15, 15-6, 11-7.

Rodriguez faces a tough clash with former world number two Maria Jose Vargas of Argentina in the last-16.