British number two Kyle Edmund was a convincing first round winner in the Citi Open, seeing off Hyeon Chung in straight sets.
Edmund, the world number 41, beat his South Korean opponent 6-3 6-1 in 57 minutes at Washington's Rock Creek Park Tennis Center.
The 22-year-old needed little more than half an hour to wrap up the first set and two early breaks in the second set him on the way to a second round match against Bulgaria's Grigor Dimitrov.
Top seed Dominic Thiem made smooth progress on a rain-affected day.
The Austrian reached the last 16 in Washington with a 6-3 6-3 victory over Switzerland's Henri Laaksonen in an hour and three minutes.
Second seed Kei Nishikori survived a tense battle against home favourite Donald Young, emerging in three sets. The Japanese eventually triumphed 6-3 4-6 7-6 (7/5) in a game that lasted two hours and 25 minutes.
Joining Thiem and Nishikori in round three is Jack Sock, who double-faulted on match point but did not waste the second opportunity to knock out Romania's Marius Copil 7-6 (7/2) 7-6 (7/5) in a game where the pair were twice forced from the court by heavy rain.
Argentina duo Juan Martin del Potro and Guido Pella progressed at the expense of Slovakia's Lukas Lacko and Germany's Mischa Zverev respectively. Marcos Baghdatis, Jordan Thompson, Dmitry Tursunov, Tennys Sandgren and Malek Jaziri.
Philipp Kohlschreiber shrugged off his recent injury scare to book his place in the second round of the Generali Open in Kitzbuhel.
The German had abruptly retired when leading compatriot Florian Mayer in their semi-final in Hamburg last week blaming tightened muscles.
But he had few problems as he battled to a 7-5 3-6 6-3 win over Argentina's world number 108 Facundo Bagnis in a match stretching over two hours.
Seventh seed Jiri Vesely saw off Yannick Hanfmann, a surprise finalist in Gstaad last week, 6-4 6-4 despite dropping his own serve once in the second set.
Sixth seed Jan-Lennard Struff beat veteran wild card Tommy Haas 6-3 7-6 (7/4), and there were also wins for Horacio Zeballos, Renzo Olivo, Gerald Melzer and Santiago Giraldo.
In the Abierto Mexicano, Spain's Fernando Verdasco beat America's Ernesto Escobedo 6-7 7-5 6-2 to book a second round meeting with France's Vincent Millot - a 6-2 3-6 6-1 conqueror of Britain's Brydan Klein.
Elsewhere, qualifier Quentin Hays upset fifth seed Ivo Karlovic 7-6 (7/5) 7-6 (10/8), Bjorn Fratangelo beat Tatsuma Ito 6-4 6-4 and Taylor Fritz came from a set down to beat Mikhail Kukushkin 6-7 (7/5) 6-2 6-2.
