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          Sloane Stephens, Rafael Nadal and an exhausting day of tennis

          World No. 1 Rafael Nadal battled for more than four hours in his third-round match against Karen Khachanov before prevailing in four sets. Julian Finney/Getty Images

          NEW YORK -- Even a 20-degree drop in the temperature and a slight drizzle Friday at the US Open could not cool off the top players.

          Defending champion Sloane Stephens played some of her best tennis of the season in a straight-sets victory against former two-time Australian Open champion (and two-time US Open finalist) Victoria Azarenka. The 6-3, 6-4 score hardly did the match justice. In the opening match of the day on Arthur Ashe Stadium, Stephens ripped her groundstrokes, which were returned with equal vigor by Azarenka.

          Stephens played the bigger points in the second set better, despite losing a couple of service games. Her stellar tennis (and her celebration after) got a standing ovation from the crowd.

          And a nice reaction from a Hall of Famer:

          After the match, Stephens said she is hoping to "take an afternoon stroll, get a hot chocolate instead of staying in the hotel for eight hours straight," given the much more palatable temperatures.

          The men's world No. 1 was tested in a big way Friday.

          Rafael Nadal and Karen Khachanov battled for 4 hours, 23 minutes, the longest match of the tournament so far and the longest of Nadal's US Open career. The highlight was a scintillating 39-rally point that Nadal won to take the third set.

          Nadal ultimately prevailed, beating the the No. 27 seed 5-7, 7-5, 7-6 (6), 7-6 (3) to reach the fourth round.

          No. 6 seed Dominic Thiem won his second-round match against America's Taylor Fritz, but not before a few outbursts. Down a set and struggling in the second, Thiem lost his cool, continuously smashing his racket on the court until you couldn't recognize what he was holding in his hand. He also appeared to be cursing (at himself) during the final set every time he made an unforced error (which was 14 total errors -- and swear words). He apologized for it during the on-court chat at the end of the match.

          "There were a lot of bad things going through my mind as everybody could see, I was very sorry for that," he said.

          A day after the pep-talk controversy involving chair umpire Mohamed Lahyani and Nick Kyrgios during the Aussie's second-round match against Pierre-Hugues Herbert, Kyrgios took to the practice courts for an hour. He looked relaxed as he prepares to face Roger Federer Saturday at noon ET on ESPN.

          Win or lose, Federer is confident there will be no pep talk from the ump like there was Thursday in Kyrgios' match. "Conversations change mindsets," Federer said Thursday to the press after his match. "That won't happen again and we all know that."

          As Nadal and Khachanov clashed on Arthur Ashe, Wimbledon runner-up Kevin Anderson was battling Canada's Denis Shapovalov on Louis Armstrong Stadium. Shapovalov saved three match points but couldn't pull through in the end.

          In news outside of Flushing Meadows, the 2014 US Open boys' champion from Australia, Omar Jasika, has been banned from playing tennis for two years after testing positive for cocaine, The Australian reported. The 21-year-old was competing in a playoff for a wild card into this year's Australian Open when the results came back. Jasika will be allowed to enter tournaments starting March 2020.