Washington - Steve Johnson of the US knocked off another seeded player in the Citi Open, beating No 9 Ivo Karlovic 3-6, 7-6 (4), 7-6 (7) to make the quarter-finals on Thursday.
Johnson topped No 5 John Isner on Wednesday.
Karlovic had 27 aces in the loss.
Seventh-seeded Kevin Anderson of South Africa also advanced, beating Malek Jaziri of Tunisia 6-3, 6-4.
In late matches, top-seeded Tomas Berdych faced Vasek Pospisil, and second-seeded Milos Raonic played Lleyton Hewitt.
In women's play, fifth-seeded Anastasia Pavlyuchenkova, sixth-seeded Svetlana Kuznetsova, and Vania King of the US won their second-round matches. Results on Thursday from the fourth day of the $1.9 million (1.4m euros) Washington Open (x denotes seed):
Kevin Anderson (RSA x7) bt Malek Jaziri (TUN) 6-3, 6-4
Steve Johnson (USA) bt Ivo Karlovic (CRO x9) 3-6, 7-6 (7/4), 7-6 (9/7)
Donald Young (USA) bt Denis Istomin (UZB) 6-3, 3-6, 6-3
Milos Raonic (CAN x2) bt Lleyton Hewitt (AUS) 7-6 (7/1), 7-6 (7/3)
Richard Gasquet (FRA x6) bt Tim Smyczek (USA) 6-3, 6-2
Vasek Pospisil (CAN) bt Tomas Berdych (CZE x1) 6-2, 6-4
Santiago Giraldo (USA) bt Victor Estrella Burgos (DOM) 6-2, 6-0
Kei Nishikori (JPN x4) bt Lukas Lacko (SVK) 6-2, 2-6, 6-3
Svetlana Kuznetsova (RUS x6) bt Kirsten Flipkens (BEL) 6-4, 7-5
Anastasia Pavlyuchenkova (RUS x5) bt Hiroko Kuwata (JPN) 6-3, 6-3
Vania King (USA) bt Christina McHale (USA) 6-1, 6-3
Ekaterina Makarova (RUS x2) bt Lauren Davis (USA) 6-2, 6-4