John Isner will play Thomaz Bellucci in the first reverse singles match on Sunday.
Robredo beat John Isner 6-7 6-3 7-6 after Sanchez saw off American Melanie Oudin 6-4 6-4.
Isner reached the last four with a battling 6-3 1-6 7-5 victory over 13th seed Simon.
Maybe this is the year John Isner barrels into the semis of a major.
Benneteau, the 2012 Sydney runner-up, will now be seeded at the Australian Open following Isner's withdrawal.
The next highest-ranked Americans are Andy Roddick at 14 and John Isner at 18.
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At 27 years of age, Isner is older than Nadal, Djokovic, and Olympic champion Andy Murray.
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Despite the defeat, Querrey is certain to usurp John Isner as the U.S.'s top-ranked male tennis player.
No surprise then, that Isner invests a huge amount of training time on improving his on-court coverage.
Elsewhere, U.S. star John Isner faces a race against time to be fit for the Australian Open.
Ferrer was followed on court by Nicolas Almagro and John Isner who played out a thrilling five-set match.
Williams and Isner combined to Anderson and Scheepers 6-3 6-2 to secure victory in the mixed team event.
Second seed Nadal reached the quarterfinals for a sixth successive year after beating 13th-seeded American John Isner 7-5 6-4.
Hewitt is almost a foot smaller than 6ft 9in Isner, but he restricted the right-hander to only five aces.
Ljubicic goes on to face David Ferrer in the last four, while Novak Djokovic will play qualifier John Isner.
Kevin Anderson beat John Isner in straight sets to draw the South Africans level before the concluding mixed doubles.
The winner of that match will play either big-serving American second seed John Isner or Swiss No. 4 Stanislas Wawrinka.
Roddick retired during last year's U.S. Open, on his 30th birthday, and now John Isner is America's No. 1 player.
Llodra ovecame 10th seed John Isner 6-4 7-6 (7-5) to end the American's hopes of making it through to London.
Roger Federer breezed past John Isner 6-3 6-4 while Novak Djokovic eased to a 6-3 6-3 victory over Ivan Ljubicic.
Elsewhere, John Isner won his career title after overcoming Spain's Nicolas Almagro in the final of the U.S. Clay Court Championship.
American 16th seed John Isner survived another marathon match as he defeated 2006 semifinalist David Nalbandian 4-6 6-3 2-6 7-6 (7-5) 10-8.
Wawrinka, the 18th seed, won 6-4 7-5 against Australian veteran Lleyton Hewitt, who on Saturday beat last year's losing finalist John Isner.
Bob and Mike Bryan's victory in Saturday's doubles rubber had put the visitors 2-1 ahead, and the much-improved Isner finished the job.
Isner looks like he's ready to fall, he's completely done and yet he still had the will to keep this thing going.
John Isner can seal America's place in the semi-finals if he beats the French No.1 Jo-Wilfried Tsonga in the first singles rubber.
Australian Leyton Hewitt also eased past big-serving American John Isner 6-2 6-4.
Murray will play Radek Stepanek after the Czech beat American John Isner 6-4 6-7 6-3 in a showdown of the big servers.
Roddick's fellow-American John Isner also reached the second round in Shanghai after saving match points in two tie-breakers against Polish qualifier Lukasz Kubot.
Isner, a winner of five titles since turning pro in 2007, has never advanced beyond the fourth round of the Melbourne grand slam.
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