WASHINGTON (AP) Trailing in a tiebreaker against top-seeded John Isner, Steve Johnson managed to put a 146 mph serve into play and then decided to try something even more audacious: lob his 6-foot-10 opponent.
Much to Johnson's surprise, the looping backhand worked, winning the point. Hanging in there against the big-serving Isner, Johnson weathered a total of 12 set points and withstood 29 aces to win their all-American quarterfinal at the Citi Open 7-6 (7), 7-6 (15) on Friday.
''You're never too tall to lob. I thought he was going to close in really hard. And I just hit a good lob. I mean, that's about it,'' the fifth-seeded Johnson said about the point that came with Isner ahead 5-3 in the opening tiebreaker. ''I don't know if I'll ever try to lob him again and it'll work, but luckily it worked today.''
Isner's take?
''I love Stevie to death,'' he said, ''but I don't think that backhand lob he it - I don't think he could do that all the time.''
Isner is a three-time runner-up at the hard-court tournament, including a year ago, when he beat Johnson in the semifinals. This time, Isner was unable to convert five set points in the opening frame and another seven in the second. When he lost the first tiebreaker on Johnson's forehand winner, Isner mangled his racket by slamming it to the court.
The second tiebreaker was even longer and more excruciating for Isner. On his second set point, at 6-4, he got a good look at a short return by Johnson but shanked a forehand long.
''I don't think he'll ever do that again,'' Johnson said. ''I don't know what happened.''
Here's how Isner explained it: ''I pulled up and got tentative.''
Not quite the longest tiebreaker of the season - Isner lost one 18-16 last month - it ended when he double-faulted for the only time in the nearly 2-hour contest to set up a sixth match point - ''so lucky,'' Johnson said later - then pushed a forehand wide.
Johnson, ranked a career-best 25th, won despite never earning a break point against Isner. But Johnson saved all six break chances Isner accumulated and wound up with 22 aces himself.
''I certainly, in my opinion, put more pressure on him that he put on me,'' Isner said.
Earlier Friday, sixth-seeded Jack Sock lost another two-tiebreaker match, beaten by 13th-seeded Ivo Karlovic of Croatia 7-6 (4), 7-6 (6).
Karlovic hit 26 aces, saved all four break points he faced and reached the semifinals in Washington for the first time since 2007. He'll play Johnson on Saturday.
The other semifinal will be second-seeded Gael Monfils against seventh-seeded Alexander Zverev. Monfils advanced by beating American Sam Querrey 6-4, 3-6, 6-1, while Zverev defeated No. 4 Benoit Paire 6-1, 6-3.
In the women's quarterfinals, top-seeded Sam Stosur, the 2011 U.S. Open champion, was surprised by 173rd-ranked wild-card entry Jessica Pegula 7-6 (4), 6-3, No. 7 Yanina Wickmayer beat No. 4 Kristina Mladenovic 4-6, 6-3, 6-4, No. 6 Yulia Putintseva eliminated Risa Ozaki 6-4, 6-2, and Lauren Davis topped Camila Giorgi 6-4, 6-4.
Sock had a chance to force a third set against Karlovic, holding a set point at 6-5 in the second tiebreaker. But Karlovic erased that with - what else? - a 133 mph ace, then delivered a 130 mph service winner to get to match point. He won when Sock's forehand clipped the net tape and flew wide.
''I feel like I scrap pretty well and get a lot of returns back and I still can't break the guy,'' Sock said. ''No matter who he plays - any round, any tournament - it comes down to a point here and there.''
The 37-year-old Karlovic, a 6-foot-11 Croatian, is coming off a title on grass at Newport, Rhode Island, last week.
He is the oldest man to win an ATP singles tournament since 1979.
Asked what it's like to enter events where he could potentially face a teenager - Zverev, for example, is 19 - Karlovic replied: ''Yeah, I mean, I could be their daddy.''
After a brief pause, he added: ''And, I mean, who knows? Maybe I am.''
Then he cracked a big smile, rocked back in his chair and said, ''No, no, I'm joking.''
Follow Howard Fendrich on Twitter at http://twitter.com/HowardFendrich