No heat rule ahead of hotly-anticipated Williams sisters matchup

US Open tennis fans were gearing up for the most anticipated match of the tournament so far on Friday with Serena and Venus Williams set to meet for the 30th time and extend a sibling rivalry that has endured for two decades.

The pair will face each other for the sixth time at their home Grand Slam, but will be spared the searing heat that has marked the opening days of the tournament when they step onto the Arthur Ashe Stadium court.

Tournament officials said there was no heat rule in effect on Friday for the first time in three days, with cloud cover expected to bring cooler temperatures.

Serena leads 16th seed Venus 17-12 in their head to head record, but most recently lost to her older sister at Indian Wells in March.

Seventeenth seed Serena, however, defeated Venus in straight sets in the 2017 Australian Open final, their last Grand Slam encounter.

With 23 Grand Slam singles titles to her name, Serena came to this year's tournament in search of a seventh US Open title.

Third-seeded Sloane Stephens will continue the defence of her US Open title against twice Australian Open champion Victoria Azarenka of Belarus, who leads her American opponent 3-2 head to head.

On the men's side, world number one and defending champion Rafael Nadal faces Russian 27th-seed Karen Khachanov in Arthur Ashe Stadium, while Argentina's Juan Martin Del Potro will be favourite in his third-round match against Spain's Fernando Verdasco having won four of their five previous encounters.

John Isner will continue his bid to end a 15-year title drought for American men when he plays un-seeded Serb Dusan Lajovic, after surviving a brutal five-set second-round match against Chilean Nicolas Jarry.

Fans and players can expect much-welcomed cloud cover and cooler temperatures, with a high of just 76 Fahrenheit (24.4 Celsius) according to the National Weather Service.

Tournament officials had imposed a heat rule on Tuesday, Wednesday and Thursday in an effort to relieve players from the sweltering temperatures, but had not implemented the measure on Friday morning.

Several players were forced to retire from play due to heat-related issues during the week, as temperatures climbed above 90 Fahrenheit (32.2 Celsius).