Argentina's Diego Schwartzman produced the latest shock at the US Open – defeating sixth seed Alexander Zverev in four sets, 3-6, 6-2, 6-4, 6-3 to reach the quarter-finals. Schwartzman had not been on court for more than two hours in any of his previous matches and the number 20 seed was by far the fresher player on a rainy day at Flushing Meadows, seeing off a weary Zverev in just over three hours. It was Labor Day in the United States and Zverev, who had seen every one of his match go to four sets or more, made hard work of his fourth-round contest, committing 17 double faults. The fatal mistake, however, came in the fourth set with Schwartzman up 4-2 and Zverev fighting to stay in the match when the German was slapped with a point penalty after directing an obscenity at his box. The 22-year-old had earlier been given a warning for slamming the ball into the crowd. Regarded as an emerging talent and potential threat to the domination of Roger Federer, Rafa Nadal and Novak Djokovic, Zverev has so far failed to meet expectations, especially at Grand Slams where he has never been beyond the quarter-finals. For Schwartzman the win was just his sixth in 28 attempts against a top-10 opponent, sending him through to the last eight at a major tournament for only the third time. Schwartzman, who cruised into the fourth round without dropping a set, saw that run end quickly when Zverev grabbed the opener despite six double faults. The Argentine had begun smartly with the early break to go up 2-0 only to have Zverev sweep the next five games on the way to taking the first set. The second began the same way with Schwartzman snatching the early break but this time he did not squander the advantage, breaking the misfiring German twice more to level the contest. Zverev had the first break in the third set but Schwartzman had the last by breaking the German at 5-4 to lead the match. Schwartzman took a stranglehold on the contest by racing to a 4-0 lead then, after a brief fight back from Zverev, finished off his opponent on his second match point with a stinging forehand winner.