Daniil Medvedev and Andrey Rublev rose through the junior ranks together but the two Russians will put their friendship on hold when they meet in the U.S. Open quarter-finals on Wednesday.
Medvedev, 24, and Rublev, 22, have been good friends since taking part in Under-10 tournaments and three years ago competed alongside each other at the Next Gen ATP Finals.
The two have soared up the ATP rankings and lead a strong Russian generation tipped to challenge for Grand Slam spoils.
Medvedev defeated Rublev in straight sets in both their previous meetings but is taking nothing for granted at Flushing Meadows.
"It's always tricky to play a friend," said Medvedev.
"It's more unconsciously, because consciously you prepare for your match, try to win.
"Unconsciously you know he's your friend since eight, know him already 16 years. There are some things going on in your mentality, but you try to get rid of it."
Third seed Medvedev, the only player left in the men's draw yet to drop a set, said Rublev made big improvements last year.
"I think he changed something in his mindset, in his game. Now he's one of the best players in the world," he added.
Rublev, seeded 10th, said he and Medvedev were "crazy on court" when they played each other as children.
"Just throwing rackets, complaining. What I remember from him he was always one of the biggest fighters I've ever seen in my life," Rublev said.
Second seed Dominic Thiem faces 21st seed Alex de Minaur of Australia in the day's other quarter-final.
In the women's draw, six-times champion Serena Williams will continue her bid for a record-equalling 24th Grand Slam title when she faces Bulgarian Tsvetana Pirokova, while former world number one Victoria Azarenka plays Elise Mertens.
(Reporting by Arvind Sriram in Bengaluru; Editing by Peter Rutherford)
