WASHINGTON -- Steve Johnson is turning into a giant killer.
Johnson knocked off another seeded, big-serving, NBA center-sized opponent in the Citi Open, beating ninth-seeded Ivo Karlovic 3-6, 7-6 (4), 7-6 (7) on Thursday to advance to the quarterfinals.
The American will face seventh-ranked and second-seeded Milos Raonic, the Canadian who beat Lleyton Hewitt 7-6 (1), 7-6 (3).
Johnson, ranked 68th, topped fifth-seeded John Isner in three sets Wednesday.
The fourth-highest ranked American on tour entering this week, Johnson fended off 27 aces by the 6-foot-11 Karlovic after the 6-foot-10 Isner had 29.
"Second day in a row (I had) to play somebody with a massive serve. I was just lucky enough to take care of business in the second and third today on my own serve and get it to two (tiebreakers)," said Johnson, who is 4-1 in tiebreakers this week. "In a tiebreaker, anything can happen."
Isner double-faulted in the third set tiebreaker to set up Johnson with match point. The same scenario played out Thursday, though not until Johnson fended off two match points to even the tiebreaker at 6-6.
"I wouldn't say that's normal, but I'm not going to complain about that," the two-time NCAA Division I singles champion said about the beneficial double faults.
Karlovic's third double fault of the match gave Johnson an 8-7 lead.
After serving out wide to Karlovic's backhand, Johnson moved in to strike the soft return. He smacked a forehand winner into the corner to reach his fifth quarterfinal in his second full season on tour.
Next up, 6-foot-5 Raonic. The Wimbledon semifinalist ranks third in aces behind Isner and Karlovic.
"They're getting shorter every day," Johnson cracked.
Seventh-seeded Kevin Anderson also advanced, beating Malek Jaziri 6-3, 6-4. American Donald Young beat No. 15 Denis Istomin 6-3, 3-6, 6-3 for his second straight victory over a seeded player.
In women's second-round play, second-seeded Ekaterina Makarova beat Lauren Davis 6-2, 6-4; fifth-seeded Anastasia Pavlyuchenkova topped Hiroko Kuwata 6-3, 6-3; sixth-seeded Svetlana Kuznetsova beat Kirsten Flipkens 6-4, 7-5; and Vania King routed Christina McHale 6-1, 6-3.