NEW YORK — The Latest on the U.S. Open tennis tournament (all times local):
Two-time major champion Garbine Muguruza let a lead slip away and was stunned in the second round of the U.S. Open by 202nd-ranked Czech qualifier Karolina Muchova 3-6, 6-4, 6-4.
The 12th-seeded Muguruza was up a set and a break in the second but faltered.
Muchova is playing in the main draw of a Grand Slam tournament for the first time.
Muguruza, in contrast, won titles at the French Open in 2016 and Wimbledon in 2017 and has been ranked No. 1.
"Somehow, I made it. I don't know how," Muchova told the Louis Armstrong Stadium crowd.
The match lasted nearly 2½ hours and ended a little past 1 a.m. on Thursday.
Defending champion Rafael Nadal's only trace of a problem during his second-round victory at the U.S. Open was a warning for letting the 25-second serve clock expire.
And Nadal was convinced that even that wasn't really his fault.
Nadal won 46 of 55 first-serve points and wore down Vasek Pospisil 6-3, 6-4, 6-2 in Arthur Ashe Stadium on Wednesday night.
The serve clocks are making their Grand Slam debut at Flushing Meadows, and folks have been curious to see whether that would have any effect on Nadal, who is known for taking his time between points. At 2-1 in the final set, he drew a warning from the chair umpire because the 25 seconds ran out. Nadal was convinced that the only reason that happened was that he paused when Pospisil briefly put a hand up to ask him to wait.
Either way, the 2-hour win was over soon thereafter.
American teenager Sofia Kenin moved into the U.S. Open's third round for the second straight time, eliminating No. 30 seed Maria Sakkari of Greece 4-6, 6-1, 6-4.
The 19-year-old Kenin, who is ranked 65th, also beat Sakkari at Wimbledon.
A year ago, Kenin received a wild-card invitation and got to the third round before losing to 2006 champion Maria Sharapova.
Jack Sock's rough year continued with a second-round loss as the No. 18 seed at the U.S. Open, beaten 4-6, 6-3, 6-2, 7-6 (3) by Nikoloz Basilashvili.
Sock, a 25-year-old American, ended a seven-match main-draw losing streak by winning in the first round.
But his loss to the 37th-ranked Basilashvili means Sock still has not won two consecutive matches all year.
Sock's record for 2018 is 6-16.
Serena Williams set up a third-round showdown at the U.S. Open against her older sister by hitting 13 aces and overwhelming 101st-ranked Carina Witthoeft of Germany 6-2, 6-2 in a little more than an hour.
This Williams vs. Williams matchup will be the earliest they've faced each other at a Grand Slam tournament in 20 years.
Venus Williams advanced to the third round with a straight-set victory earlier Wednesday.
Serena leads Venus 17-12 overall and 15-10 at majors.
John Isner reached the U.S. Open's third round for the seventh consecutive year, hitting 38 aces to come back and edge 42nd-ranked Nicolas Jarry of Chile 6-7 (7), 6-4, 3-6, 7-6 (2), 6-4.
The 11th-seeded Isner is the highest-ranked American man in New York for the seventh time in a row, but he's only once made it to the fourth round in that span.
He broke Jarry for the second and last time at 4-all in the fifth set, flicking a backhand lob winner to take that game.
After serving out the win, Isner walked over and slapped palms with front-row spectators.
Fresh off her first Grand Slam semifinal, No. 9 seed Julia Goerges made an early exit at the U.S. Open.
Goerges bowed out in the second round at Flushing Meadows, losing to 45th-ranked Ekaterina Makarova 7-6 (10), 6-3.
While their respective rankings make this seem to be a big upset, Makarova actually has had more success at hard-court major tournaments than Goerges, reaching semifinals at the 2014 U.S. Open and 2015 Australian Open.
Goerges had never been past the fourth round at any Grand Slam tournament until her run to the final four at Wimbledon last month.
Andy Murray's return to the U.S. Open lasted just two rounds.
The 2012 champion was eliminated by No. 31 seed Fernando Verdasco 7-5, 2-6, 6-4, 6-4. The Spaniard will face a second straight past champion at Flushing Meadows when he meets third-seeded Juan Martin del Potro in the third round.
Murray missed last year's tournament because of a hip injury and didn't consider himself a contender this year because of his lack of preparation.
Verdasco won for just the second time in 15 career meetings with Murray, who had multiple chances to even the final game of a 64-minute final set before the left-hander finished it.
Juan Martin del Potro moved on to what could be a third-round matchup of past U.S. Open champions by beating American Denis Kudla 6-3, 6-1, 7-6 (4).
The No. 3 seed from Argentina was the 2009 champion. He could next face Andy Murray, who won his first Grand Slam title in 2012 at Flushing Meadows.
Murray was playing No. 31 seed Fernando Verdasco in the second round.
Venus Williams has done her part to set up an all-Williams matchup at the U.S. Open. Now it's her sister's turn to make it happen.
The 16th-seeded Williams reached the third round at Flushing Meadows for the fifth year in a row, beating 40th-ranked Camila Giorgi of Italy 6-4, 7-5 despite producing fewer than half as many winners.
If Williams' younger sister, Serena, can beat Carina Witthoeft in the second round on Wednesday night, the two siblings with a combined 30 Grand Slam titles will play each other Friday.
It would be the earliest Williams vs. Williams showdown at a Grand Slam tournament since their very first tour-level match, all the way back at the 1998 Australian Open.
Venus says: "I hope we get to play."
Of the 82 points that went her way Wednesday, only 13 came via her own winners. Giorgi had 29 winners, but also 41 unforced errors and 28 forced errors.
Defending champion Sloane Stephens rallied to reach the third round of the U.S. Open with a 4-6, 7-5, 6-2 victory over Anhelina Kalinina.
Stephens set up a third-round matchup with Victoria Azarenka, the former No. 1 player who has twice been a U.S. Open finalist.
Stephens served an ace to finally put away the 2-hour, 46-minute match against Kalinina, a Ukrainian qualifier who was playing her first Grand Slam tournament.
Stan Wawrinka ran his U.S. Open winning streak to nine with a 7-6 (5), 4-6, 6-3, 7-5 victory over qualifier Ugo Humbert of France.
Wawrinka, the 2016 champion who missed the Grand Slam tournament last year because of two left knee surgeries, advances to face No. 25 seed Milos Raonic of Canada in the third round.
Raonic swept past Gilles Simon of France 6-3, 6-4, 6-4.
Defending U.S. Open champion Sloane Stephens has forced a third set in her second-round match.
The No. 3 seed lost the first set 6-4 to qualifier Anhelina Kalinina of Ukraine but battled back to take the second set 7-5.
The winner faces two-time U.S. Open finalist Victoria Azarenka.
No. 15 seed Stefanos Tsitsipas has been eliminated in the second round of the U.S. Open, a surprisingly early exit after his strong hard-court season.
The 20-year-old Greek was beaten by Daniil Medvedev of Russia, 6-4, 6-3, 4-6, 6-3.
Tsitsipas beat four top-10 opponents in Toronto — becoming the youngest player to do so at an event since the ATP World Tour began in 1990 — before losing to top-ranked Rafael Nadal in the final. He also reached the semifinals in Washington before a loss to No. 4 Alexander Zverev, and came into the U.S. Open as the highest-seeded debutant since Richard Gasquet was No. 13 in 2005.
But he had 64 unforced errors against just 27 winners in the match that lasted 2 hours, 48 minutes.
Victoria Azarenka easily reached the third round in her return to the U.S. Open, beating No. 25 seed Daria Gavrilova of Australia 6-1, 6-2.
The former top-ranked player could meet defending champion Sloane Stephens in the third round.
Azarenka, a two-time U.S. Open finalist, is playing in the tournament for the first time since 2015 after giving birth to a son.
With a ranking too low for direct entry, she received a wild card into the tournament. She is unseeded at Flushing Meadows for the first time since 2007.
The U.S. Open says it regrets that a player was given a code violation for changing her shirt on the court during a match, clarifying its rules on when that is allowed.
Alize Cornet of France returned from a break in action during her match Tuesday with her shirt on backward. She took off the shirt and put it back on correctly while standing near the back of the court and was given a code violation by the chair umpire.
The penalty drew criticism on social media, with people — including Andy Murray's mother, Judy — noting that men frequently change their shirts while sitting in their chairs during changeovers.
Tournament officials said in a statement Wednesday that all players can change their shirts when sitting in the player chair, adding that female players can also change their shirts in a private location near the court if it is available without being assessed a bathroom break.
Play at the U.S. Open is beginning under an extreme heat policy and it will remain in effect until further notice.
The women's tour has a heat rule that allows for breaks during singles matches because of the heat. The men's tour does not, but the policy was extended to the men on Tuesday when temperatures soared into the mid-90s. They are expected to remain there Wednesday.
The WTA Tour rule permits a 10-minute break between the second and third sets if either player requests it. The men's break would come between the third and fourth sets.
A statement from the U.S. Tennis Association says appropriate medical timeouts for heat-related illness also are allowed.
Venus and Serena Williams can set up their earliest Grand Slam meeting in 20 years if both win second-round matches at the U.S. Open on Wednesday.
Venus has an afternoon match on another hot day in Flushing Meadows against Camila Giorgi. Serena, at No. 17 seeded one spot below her older sister, should face cooler conditions during a night match against Carina Witthoeft.
If both siblings win, their third-round matchup would be their earliest at a Grand Slam tournament since they played in the second round of the 1998 Australian Open in their very first meeting on tour.
Temperatures were expected to be in the mid-90s, as high as or even a little warmer than Tuesday, when six men retired from their matches.
Defending champions Rafael Nadal and Sloane Stephens also are in action, along with past champions Juan Martin del Potro and Andy Murray — who could meet in the third round.