WASHINGTON: Top seeds John Isner and Samantha Stosur were ousted from the ATP and WTA Washington Open in Friday’s quarterfinals, 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.

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 semifinal after the American beat Italy’s Camila Giorgi 6-4, 6-4. The other semifinal 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 semifinal 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 quarterfinal 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.

Venus in Stanford semis

In San Francisco, top-seeded Venus Williams booked her semifinal berth at the WTA hardcourt tournament in Stanford, California, on Friday with a 6-4, 6-1 victory over 17-year-old wild card CiCi Bellis.

Ranked seventh in the world, the 36-year-old Williams needed only an hour to get past the 203rd-ranked Bellis in an all-American clash.

Williams, who captured the most recent of her 49 singles titles in February in Taiwan, next faces another American, Alison Riske, who was leading 6-3, 0-1 when fourth-seeded Coco Vandeweghe retired from their quarterfinal with a right ankle injury.

Vandeweghe left the court in a wheelchair, pressing an ice pack to her ankle.

Bellis, playing in her first WTA quarterfinal was buoyed by an early service break.

But former world No.1 Williams quickly regained the break and broke again with a backhand winner down the line to secure the opening set.

It was all Williams in the second set as she moved a step closer to a third Stanford title to go with those she won in 2000 and 2002.

The other semifinal will pit second-seeded Dominika Cibulkova of Slovakia against third-seeded Johanna Konta of Britain.

Cibulkova won 11 straight games after trailing 2-5 in the opening set en route to a 7-5, 6-0 victory over Japanese fifth seed Misaki Doi.

Konta defeated China’s Zheng Saisai 6-2, 4-6, 6-3.

Cibulkova’s victory ensures she will return to the top 10 in the world rankings for the first time in more than 18 months.

Her 2016 campaign has included two titles and a run to the quarterfinals at Wimbledon.