Washington - Top seeds John Isner and Samantha Stosur were ousted from the ATP and WTA Washington Open in Friday's quarter-finals, each paying the price for squandering leads in tie-breakers.
Isner unleashed a 29-ace barrage at US fifth seed Steve Johnson but failed to capitalize on five set points in the first set and seven in the second in falling 7-6 (9/7), 7-6 (17/15).
"Those were very easily tie-breakers I could have won but I didn't," Isner said. "I had countless chances. I did a lot of really great things. I put more pressure on him than he did on me. It didn't pay off."
Isner made his first double fault on the penultimate point, then smacked a forehand wide to hand Johnson the victory after an hour and 57 minutes, the last 23 minutes of it in the tension-packed tie-breaker.
"I shot myself in the foot in the second set," Isner said.
Johnson, ranked a career-high 25th, next faces another serve smasher in 37-year-old Croatian Ivo Karlovic, who blasted 26 aces to dispatch US sixth seed Jack Sock 7-6 (7/4), 7-6 (8/6).
"In the crucial moments I was able to come up with my best shots," Karlovic said. "Hopefully I can use this win for confidence to play well."
Australia's Stosur, the 2011 US Open champion, lost 7-6 (7/4), 6-3 to US wildcard Jessica Pegula after dropping the last six tie-breaker points and losing the only break point of the second set.
"I was up 4-1 in the tie-breaker and let that go. Probably that was it," Stosur said. "A lot came down to a few points. As soon as one person got one clean shot the point was over."
Pegula, at 22, was 10 years younger than Stosur and at 173rd in the world was 159 spots below the Aussie in the rankings. She was aided by a practice Sunday with Stosur.
"That was probably the biggest win of my career," Pegula said. "I'm really excited. I've been playing really well. I put in a lot of hard work and it paid off. It really helped me this week practicing with her."
Pegula will face 122nd-ranked Lauren Davis in a semi-final after the American beat Italy's Camila Giorgi 6-4, 6-4. The other semi-final sends Belgian seventh seed Yanina Wickmayer against Kazakh sixth seed Yulia Putintseva.
French second seed Gael Monfils avoided the upset bug by downing US eighth seed Sam Querrey 6-4, 3-6, 6-1. His semi-final foe will be seventh-seeded German 19-year-old Alexander Zverev, who eliminated French fourth seed Benoit Paire 6-1, 6-3.
"I like him. It's going to be a big match," said Monfils, who is 2-0 against Zverev, including a February quarter-final win at Rotterdam.
Zverev, ranked 27th, is the youngest player in the top 30 since Rafael Nadal in 2005. He reached his first two ATP finals in Halle and Nice over the past two months.
Monfils, 29, seeks a sixth career title, his first since 2014 in Montpellier, his first ever title outside of Europe and first outdoor crown since Sopot in 2005.
Johnson, who captured his first ATP title last month at Nottingham, fired 22 aces of his own, denied Isner on six break points and took his second win in six meetings with Isner.
"He was serving extremely well," Johnson said. "I didn't have any break points. I was able to scrap out the first set. I got very lucky when he double faulted at the end and I was able to close it out."
Karlovic, who matched his best Washington run from 2007 by reaching the semi-finals, won his seventh ATP title last week on Newport grass to become the oldest tour singles champion since 1979.
"John had 29 aces and Ivo could have as many tomorrow," Johnson said. "You just have to focus on your serve, try and get to tie-breaker and take your chances."
Johnson is 3-1 against 35th-ranked Karlovic, including a third-round victory at Washington in 2014.
No American has won the Washington title since Andy Roddick in 2007.