Daniil Medvedev edged a battle of up-and-comers at the Citi Open, while Donald Young also progressed to the second round.
Medvedev, 21, sent down 15 aces to get past 19-year-old Reilly Opelka 6-7 (8-10) 6-3 7-6 (8-6) in a marathon battle in Washington.
The Russian, who stunned Stan Wawrinka in the first round at Wimbledon, saved a match point in the third-set tie-break before converting his first such opportunity.
Young had to come from behind to advance, getting past wildcard Tim Smyczek 4-6 6-3 6-4.
No seeds were in action in Washington, with Guido Pella, Nicolas Mahut, Jared Donaldson, Marius Copil and Lukas Lacko also among the winners.
At the ATP 250 event in Los Cabos, Adrian Mannarino came through his opener.
Mannarino, the French seventh seed, was too good for Jason Jung in a 6-3 6-3 victory.
However, fellow seed Frances Tiafoe had no such luck, edged by Australian wildcard Thanasi Kokkinakis 7-6 (7-3) 7-6 (12-10).
Qualifiers Matthew Ebden, Evan King and Akira Santillan also had good days, winning through.
Santillan needed just 36 minutes to crush Konstantin Kravchuk 6-0 6-1.