Milos Raonic and Bernard Tomic had straight-sets wins while Sam Querrey needed a long third-set tiebreaker on Tuesday to move into the last 16 at the Citi Open in Washington.
Canadian Raonic, seeded fourth, beat qualifier Sam Groth 7-5, 6-4 while Groth's Australian compatriot Tomic breezed past Belgian David Goffin 6-1, 6-3.
It was a far tougher assignment for American Querrey, who edged Uzbek Denis Istomin 4-6, 6-3, 7-6 (9).
"It was stressful out there," said Querrey, who began his postmatch news conference with a hearty yawn. "Just happy to get through it."
Querrey's opponent in the round-of-16 will be Bulgarian Grigor Dimitrov, defeated Belgian veteran Xavier Malisse 7-6 (4), 6-4.
One seeded player who will be going no further is Ukraine's Alexandr Dolgopolov, who was beaten 6-3, 7-6 (4) by India's Somdev Devvarman.
In first-round matches, former world No. 1 Lleyton Hewitt lost 6-3, 7-5 to America's Ryan Harrison and local wildcard Jack Sock beat Croatia's Igor Sijsling 6-4, 6-2, setting up a second-round clash with second-seeded Kei Nishikori of Japan.
On the women's side, top-seeded Angelique Kerber cruised past Irina Falconi 6-2, 6-3 in the first round.
With No. 15-ranked Sloane Stephens' loss Monday, No. 9-ranked Kerber is 17 spots higher than any player remaining in the tournament. The German will face the winner of Melanie Oudin and Anastasia Rodionova in the next round.
Kerber was granted a wild card into the Citi Open, and Falconi was a qualifier.
Wild card Taylor Townsed lost to Monica Niculescu 6-3, 6-0, while No. 4-seeded Alize Cornet downed Yanina Wickmayer 6-1, 6-3, sixth-seeded Mona Barthel cruised past Stefanie Voegele 6-2, 6-2.
No. 5-seeded Sorana Cirstea advanced when Lesia Tsurenko retired while trailing 7-5, 1-1 and Madison Keys beat Michelle Larcher de Brito 7-5, 6-3.