Washington - Third-ranked defending champion Alexander Zverev took bragging rights in his sibling rivalry, beating 15th-seeded older brother Mischa 6-3, 7-5 on 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," Alex said. "Hopefully the first of many. I hope we meet again someday in a final."
The rain-hit US Open tune-up 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 tune-ups 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.
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 on Thursday from the fourth day of the ATP and WTA Washington Open (x denotes seeding):
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
Andrey Rublev (RUS x16) bt Noah Rubin (USA) 6-3, 6-2
Kei Nishikori (JPN x7) bt Denis Shapovalov (CAN x9) 7-6 (7/1), 6-3
Alex De Minaur (AUS) bt Chung Hyeon (KOR x8) 6-2, 4-6, 6-2
Denis Kudla (USA) bt Lucas Pouille (FRA x6) 7-5, 6-7 (6/8), 7-6 (7/5)
Andy Murray (GBR) bt Marius Copil (ROM) 6-7 (5/7), 6-3, 7-6 (7/4)
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)
Belinda Bencic (SUI x6) bt Nao Hibino (JPN) walkover
Zheng Saisai (CHN) bt Ysaline Bonaventure (BEL) 6-4, 6-4
Allie Kiick (USA) bt Katie Boulter (GBR) 6-4, 5-7, 6-1
Donna Vekic (CRO x7) bt Fanny Stollar (HUN) 6-2, 3-6, 7-6 (15/13)
Svetlana Kuznetsova (RUS) bt Ana Bogdan (ROM) 6-2, 6-1
Yulia Putintseva (KAZ x8) bt Jen Brady (USA) 7-5, 6-2
Magda Linette (POL) bt Naomi Osaka (JPN x3) 6-2, 3-6, 6-3