In the latest example of a player fined thousands of dollars at a Grand Slam tournament for what tennis officials deem a lack of effort, 28th-seeded Carla Suárez Navarro of Spain was docked $40,000 at the U.S. Open on Thursday, two days after retiring from her first-round match with a lower-back injury.

She was due to collect $58,000 as a first-round loser in singles at Flushing Meadows.

But tournament referee Soeren Friemel said Suárez Navarro, who can appeal the decision, "did not perform to the required professional standards" and so was punished for violating the first-round performance rule. She stopped playing after losing the first set of her match against Timea Babos by a 6-2 score.

The Grand Slam Board introduced the rule before the 2018 season to deter players who enter tournaments while injured from retiring during first-round matches.

Suárez Navarro also retired from a match at the hard-court tournament in Toronto earlier this month.

"Not the easiest weeks for me dealing with some back pain," she tweeted Thursday. "We made our best effort to be ready and play our heart out, but it got really worse during my opening match in New York."

In July, American player Anna Tatishvili was awarded her French Open prize money when the Grand Slam Board reversed her fine of about $50,000 for a 6-0, 6-1 loss at the French Open. That was her first tournament since October 2017.

Bernard Tomic was fined his full prize money of about $55,000 at Wimbledon last month after a three-set loss in the first round that lasted only 58 minutes.

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DOUBLE TROUBLE

Naomi Osaka would be happy to play mixed doubles with Kei Nishikori for Japan at the 2020 Toyko Olympics.

Just one problem: The reigning U.S. Open singles champion apparently isn't much of a doubles player.

After Osaka's victory in singles Thursday — with Colin Kaepernick and Kobe Bryant watching from her courtside box — she said that "anyone that knows my doubles track record" knows she's not exactly an expert.

She hasn't played a tour-level women's doubles match since 2017 ... and she went 0-5 that season.

Nishikori said last week he was planning to play singles and men's doubles in Tokyo but wasn't sure about mixed, because it might mean too much tennis in the hot and humid weather. But, he added he would talk to Osaka at some point.

They are friends, and Osaka said their pairing in Tokyo would be "historic."

"I would definitely play with him," Osaka said. "I just would actually need to practice doubles for the first time in my life. Because you cannot play mixed doubles with Kei Nishikori and lose in the first round of the Olympics in Tokyo. That would be the biggest — like, I would cry. I would actually cry for losing a doubles match."

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SET-DOWN SERENA

That Serena Williams came back to beat 17-year-old Caty McNally at the U.S. Open after dropping their opening set shouldn't have come as a surprise: No one in the history of professional tennis has been as good as Williams at that sort of turnaround.

Turns out Williams actually wins more Grand Slam matches than she loses after trailing by a set.

Her 5-7, 6-3, 6-1 victory over McNally in the second round at Flushing Meadows improved Williams' career record at major tournaments to 42-40 after being down a set — the only woman in the 50-year Open era who can boast of a winning record in such situations, according to the WTA.

The only other active player with more than 25 such victories is her older sister, Venus, who has 28.

In all tour-level main-draw matches, Williams is 97-107 after ceding the opening set, a .475 winning percentage that leads active players. Next on the list? Maria Sharapova at .390, 83-130.