Third-ranked defending champion Alexander Zverev took bragging rights in his sibling rivalry, beating 15th-seeded older brother Mischa 6-3, 7-5 Thursday to reach the ATP Washington Open quarter-finals.

The German siblings met for the first time in their tour careers, the 21-year-old younger brother besting Mischa, who turns 31 later this month, as he rarely did in boyhood battles.

“It was unbelievable,” Alexander said. “Hopefully the first of many. I hope we meet again someday in a final.”

The rain-hit US Open tuneup event also saw US second seed John Isner ousted by 152nd-ranked compatriot Noah Rubin 6-4, 7-6 (8/6).

Three-time Grand Slam champion Andy Murray of Britain, in his first hardcourt event after an 11-month layoff with a right hip injury, was set for a later match against Romanian Marius Copil.

Alex broke Mischa for a 5-3 lead and held again to win the all-Zverev first set in 29 minutes.

After exchanging breaks early in the second set, Alex saved a break chance in the 11-minute eighth game, then waited through a 15-minute rain delay and denied another break point with a service winner in holding to reach 5-5.

Alex broke again in the penultimate game and held once more to win after one hour and 51 minutes on a forehand crosscourt drop volley winner that had Mischa running to the net.

The elder Zverev walked around the net and shared a hug with Alex, patting his practice partner on the back as they parted for their chairs.

“I actually played better than in practice,” Mischa said. “We had a few great rallies. It was just fun.

“When he hits certain shots, I know what he’s thinking. But that’s a disadvantage, too, because he knows what I’m thinking.”

Mischa won his first ATP title in June at Eastbourne, a feat he credits his brother for helping him to achieve.

“It was something I always wanted,” Zverev said. “It was him being young and being overly positive that helped me be young, keep working hard and achieve my dream. When I think about it I still get emotional.”

Ninth-ranked Isner, fatigued after a first-ever Grand Slam semi-final at Wimbledon and an Atlanta title run, had beaten Rubin in the first round of the French Open.

Isner blamed his low-energy outing in part on his busy recent schedule and also on the showers that delayed his opener to Thursday.

“For me, this could very well be a good loss. I need a little rest,” Isner said. “There are some big tournaments coming up. I want to be full of energy for those.”

Isner plays at Canada and Cincinnati the next two weeks in major tuneups for the US Open, which starts August 27.

Isner, who also won the Miami Open in March, surrendered a break in the penultimate game of the opening set. Rubin denied Isner on 7-of-8 break points while converting both of his break chances to win the first set.

“Noah played with more energy. He was just tougher than I was. He was hungry out there,” Isner said. “I’ve felt better but I’ve also won matches when I’ve been a little low on fuel.”

Isner never managed another break point against Rubin, who won after 99 minutes when Isner netted a forehand.

Rain ‘tires you out’

A fourth day of showers delayed their start by 2 1/2 hours.

“It’s unfortunate the rain has wreaked havoc so far. Noah dealt with it better than I did,” Isner said. “It tires you out. It’s mentally taxing. You’re sitting around hours on end.”

Isner lost an epic 26-24 fifth set to South African Kevin Anderson in the Wimbledon semi-finals. It was the second-longest match in Grand Slam history, lasting six hours and 36 minutes.

Rubin advanced to a later third-round match against Russian 16th seed Andrey Rublev, who eliminated American Tommy Paul 7-6 (7/5), 6-4.

“If I would have won, I don’t know what I would have done,” Isner said.

Greek 10th seed Stefanos Tsitsipas, who in Barcelona became his homeland’s first ATP singles finalist in 45 years, reached the quarter-finals by beating Australian James Duckworth 6-3, 6-4.

Results

3rd rd

Stefanos Tsitsipas (GRE x10) bt James Duckworth (AUS) 6-3, 6-4

Alexander Zverev (GER x1) bt Mischa Zverev (GER x15) 6-3, 7-5

David Goffin (BEL x3) bt Frances Tiafoe (USA x13) 6-0, 6-3

2nd rd

Alex De Minaur (AUS) bt Steve Johnson (USA) 6-4, 7-5

Andrey Rublev (RUS x16) bt Tommy Paul (USA) 7-6 (7/5), 6-4

Noah Rubin (USA) bt John Isner (USA x2) 6-4, 7-6 (8/6)