WASHINGTON — After playing doubles past midnight and getting to sleep at about 3 a.m., Steve Johnson was less than thrilled to be playing in the first Citi Open men's match in the main stadium at 4 p.m. Thursday.

Still, showing the sort of progress that has lifted his ranking to a career-high 25th, Johnson played quality first-strike tennis and ended qualifier Ryan Harrison's season-best run by winning the all-American matchup 6-4, 6-4 to reach the hard-court tournament's quarterfinals.

"I was lucky enough to be on the schedule first today. Just good turnaround," the fifth-seeded Johnson said, his words thick with sarcasm. "Got a few hours of sleep. Just get back up. That's life. Hopefully, the ATP and those guys don't do that much anymore, because I don't think it's fair. It's up to those guys. They've got to do what they've got to do."

Tournament director Jeff Newman said the scheduling was affected by rain that washed out play earlier in the week and court assignments are based on various factors such as "a television perspective, a fan perspective, how the draws are matched up."

"Every player wants to play in a certain position. You try to take feedback and weigh it all," Newman said. "Some players will be happy; some people would prefer other spots."

Johnson, a member of the U.S. Olympic team headed to Rio de Janeiro next month, broke Harrison once in each set, did not face a break point himself and ended the match with a 130 mph ace.

A two-time NCAA singles champion in 2011 and 2012, and a member of Southern California teams that won four consecutive national titles, Johnson earned his first ATP trophy at Nottingham, England, last month, then backed that up by making his fourth-round debut at a Grand Slam tournament at Wimbledon.

"I knew I was capable always of winning a title — and, hopefully, multiple titles," Johnson said. "Once you win it, then you know what it feels like, you know what it's like to be serving for a title. You get to those situations a little bit more and you just hopefully become more comfortable."

On Friday, Johnson will face top-seeded John Isner, who hit serves at up to 143 mph, produced 16 aces and never faced a break point in a 7-6 (3), 6-2 victory over No. 15 Marcos Baghdatis. This will be the third year in a row that Americans Johnson and Isner meet in Washington; Johnson won in 2014, and Isner did in 2015, both decided by a third-set tiebreaker.

"He'll tell you this much: He was playing pretty poorly the first five months of the year. Wasn't getting much going. But he turned the corner (in June)," Isner said. "He's carried that confidence over to here. He's just like anyone else: When he gets confidence on his side, he starts going for his shots and his shots start finding the right spots."

Also Thursday, No. 2-seeded Gael Monfils overwhelmed No. 16 Borna Coric 6-2, 6-3, No. 13 Ivo Karlovic hit 21 aces and eliminated No. 3 Bernard Tomic 7-6 (4), 6-3, No. 6 Jack Sock beat Dan Evans 6-1, 7-5, No. 7 Alexander Zverev got past Malek Jaziri 6-2, 5-7, 6-2, and No. 8 Sam Querrey took the last five games in a 7-6 (5), 6-3 victory over 2012 champion Alexandr Dolgopolov.

In women's action, third-seeded Monica Puig lost to Lauren Davis 6-4, 6-2, fourth-seeded Kristina Mladenovic beat 2013 Wimbledon runner-up Sabine Lisicki 6-3, 6-3, Risa Ozaki edged Naomi Broady 6-1, 5-7, 6-4, and Camila Giorgi advanced when Tamira Paszek withdrew before their match because of an upper respiratory infection.

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