All four seeded players in action Tuesday at the Citi Open in Washington survived the round of 32, although No. 7 Karen Khachanov and No. 8 Botic van de Zandschulp needed three hard-fought sets to defeat qualifiers.
Khachanov, of Russia, defeated Germany's Dominik Koepfer 6-7 (3), 7-5, 6-4 in second-round play. Khachanov, who is ranked 24th in the world, advanced in part by recording nine aces to Koepfer's five and double-faulting only three times to Koepfer's seven.
van de Zandschulp, of the Netherlands, had an even more difficult time with Croatia's Borna Gojo, 7-6 (6), 6-7 (5) 6-4, in a match that lasted three hours and seven minutes.
Gojo survived a match point in the second set and forced a tiebreaker, then won that when he won four points van de Zandschulp's serve.
Elsewhere in the second round, top-seeded Andrey Rublev of Russia, who is ranked eighth in the world, put away Great Britain's Jack Draper 6-4, 6-2.
In a matchup of Brits, the 16th-seed, Daniel Evans, needed just over an hour to defeat Kyle Edmund 6-1, 6-2.
Australia's Nick Kyrgios won his first-round match, topping the United States' Marcos Giron 6-3, 6-2.
Abierto de Tenis Mifel
In Los Cabos, Mexico, eighth-seeded Tomas Martin Etcheverry of Argentina won his first set against Australia's Jason Kubler before dropping the final two and the first-round match, 6-7 (4), 6-3, 6-1.
Wild-card entrant Alex Hernandez of Mexico advanced by beating Nicolas Barrientos of Colombia 3-6, 6-4, 6-4.
Two more seeded players, No. 6 Brandon Nakashima of the U.S. and No. 7 Thanasi Kokkinakis of Australia, were set to play later Tuesday night. Nakashima was to face Japanese qualifier Kaichi Uchida, while Kokkinakis was to meet Spain's Fernando Verdasco.
--Field Level Media