The measure to allow breaks in matches was implemented for three days last week as the players battled sopping humidity and scorching heat.
Temperatures were forecast to hit up to 90 degrees Fahrenheit (32.2 degrees Celsius) but the conditions did not deter fans flocking to Flushing Meadows for day eight of the tournament, as former champions Novak Djokovic, Roger Federer and Maria Sharapova bid to book their quarter-final spots.
Twice winner Djokovic will meet unseeded Joao Sousa, who is appearing in the fourth round of a Grand Slam for the first time, on Arthur Ashe stadium. The sixth-seeded Serbian leads Sousa 4-0 in head-to-heads, most recently walloping him in straight sets in the second round of last year's French Open.
Second seed Federer looks to have an equally smooth road to the quarters as he faces unseeded Australian John Millman.
Elsewhere, rising stars will look to make an impact.
American Madison Keys, last year's runner-up, starts the action on Arthur Ashe stadium against speedy Slovakian Dominika Cibulkova, who upset fourth seed Angelique Kerber.
Twentieth seed Naomi Osaka, who secured straight-sets victories in her first three matches, meets 26th seed Aryna Sabalenka.
Osaka is the first Japanese woman to reach the tournament's fourth round since 2004 and has won 22 consecutive games en route to the match against her fellow 20-year-old.
(Reporting by Amy Tennery, editing by Ed Osmond)
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