Mirra Andreeva, of Russia, left, helps Anastasia Potapova, of Austria. (AP Photo)

NEW YORK CITY: Anastasia Potapova clutched her knee and crumpled to the court in the closing stages of a gruelling US Open fourth-round clash. After two hours and 25 minutes of pushing each other to their limits, the moment the 25-year-old went down, Mirra Andreeva, the No.5 seed, sprinted across the court to help her friend.In the end, Andreeva, the reigning Roland Garros champion, won the last game to seal a 5-7, 6-4, 6-3 victory, but it was her rush to Potapova’s aid that lingered long after the match.Andreeva, 19, first met Potapova in Monastir, Tunisia when she was handed a wildcard into the WTA 250 and drew Potapova in the first round. At the time, she was already an established name on tour and, to Andreeva, a player she had watched from afar.“Nastya was always like a prodigy. She came on tour early, she also had a lot of wins,” Andreeva recalled.Potapova won that first meeting, but Andreeva came away impressed by more than her tennis.“She said a lot of nice words,” Andreeva said. The two began practising together and, as they spent more time around each other, their friendship grew naturally.In the quarterfinals, Andreeva will play world No.4 Coco Gauff.