Japanese outsider Kei Nishikori stole the spotlight on another day of shocks at the US Open on Saturday, while Venus and Serena Williams rolled towards a quarterfinal showdown at Flushing Meadows.

Fourth seed David Ferrer was left a punched out wreck as he went down 4-6, 4-6, 6-3, 6-3, 5-7 following an inspired performance by Nishikori.

The 18-year-old made his ranking of 126 look like a mathematical error during the 3-1/2 hour tussle and he kept his nerve to become the first Japanese man since 1937 to reach the last 16 in New York.

"I still can't believe it. I was playing great and he was playing great too. That was the biggest win for me," beamed the Japanese, who collapsed on to his back after sealing victory.

He will next face rising Argentine Juan Martin Del Potro, who tamed French 16th seed Gilles Simon 6-4, 6-7 (4), 6-1, 3-6, 6-3.

Rafael Nadal of Spain pumps his fist after a point against Viktor Troicki of Serbia during their match at the U.S. Open tennis tournament at Flushing Meadows in New York, August 30, 2008. [Xinhua/Reuters]

Rafael Nadal can expect an easy ride into the semifinals after his draw opened up following the defeats of seventh seed David Nalbandian, ninth seed James Blake and 14th seed Ivo Karlovic.

Argentina's Nalbandian was ousted by 32nd seeded Frenchman Gael Monfils 3-6, 4-6, 2-6 and hard-serving Karlovic was beaten 6-7 (5), 6-7 (5), 2-6 by American Sam Querrey.

Mardy Fish completed a bad night for the seeds when he trampled over friend and fellow American Blake 6-3, 6-3, 7-6 (4).

Andy Murray, the British sixth seed, also looked to be on his way out when he was two points from defeat before he found his range to pull off a 6-7 (5), 4-6, 7-6 (5), 6-1, 6-3 win over Austrian Juergen Melzer.

World No. 1 and Wimbledon champion Nadal, who will not face a top 30 player until at least the semifinals, blasted into the fourth round by grinding down Serbian Viktor Troicki 6-4, 6-3, 6-0.