Daniel Murphy went 2 for 4 for New York and is batting .500 (14 for 28) in his last seven games.
The Spaniard, the 23rd seed, was leading Brazilian Marcos Daniel 7-6 2-2 when he decided he was unable to continue.
Defending champion Albert Montanes moved into the last eight with a 6-1 6-2 victory against fellow Spaniard Daniel Gimeno-Traver.
Del Potro outclassed World No.97 Daniel Brands 6-4 6-2 while Berdych overpowered Russian wild card Dmitry Tursunov 6-3 6-2 in just over an hour.
After a basket by Tech's Daniel Miller's made it 69-all with 2:14 left, each team misfired.
Third seed Daniel Hantuchova, of Slovakia, gained a 6-2 6-4 victory over Olga Govortsova, of Belarus, while fourth seed Na Li , of China, overcame Russian Yaroslava Shvedova 4-6 6-2 6-0.
Nadal will meet Jurgen Melzer next, after the Austrian overcame Spain's Daniel Gimeno-Taylor 6-3 7-6 (7-2).
Conservative MP Daniel Kawczynksi made a speech in which he told MPs that more than 2, 000 cattle were slaughtered in Shropshire last year as a result of bovine TB.
Djokovic, the 2008 champion, defeated world No. 74 Daniel Gimeno-Traver of Spain 7-5 6-3 6-2 to set up a clash with Marco Chiudinelli of Switzerland.
In the final, the Bryans whipped the world's No. 2 team and arguably its best on fast courts, Max Mirnyi and Daniel Nestor.
Ms Alexander's solicitor, Daniel Machover, says failure to hold an inquest would be a breach of her right under Article 2 of the European Convention of Human Rights to have a prompt investigation into her son's death.
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