By Ann Liguori
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Two American hopefuls — eight-seeded Madison Keys and 25th-seeded Jack Sock — could not get it done in their fourth-round matches at the US Open on Sunday afternoon. So the only Americans left in the field are Serena Williams and Venus Williams.

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As I wrote in my last column, where would American tennis be without the Williams sisters?

On Sunday afternoon, Caroline Wozniacki beat Keys 6-3, 6-4. And as clean of a match as Wozniacki played (only seven unforced errors), Keys didn’t help her cause with 33 unforced errors. Keys smacked 17 forehand winners, but she misses a lot. I find it agonizing to watch her sometimes because she makes so many unforced errors. She either crushes the ball or not. There doesn’t seem to be another gear.

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Yes, the 21-year-old showed resilience in her third-round match when she rallied back from a 5-1 deficit in the third set to beat Japanese teenager Naomi Osaka. Osaka helped Keys’ cause in that match, getting tight and making her own high number of unforced errors. She even double-faulted on match point.

And yes, Wozniacki is a former No. 1 and a two-time runner-up, and she is experiencing a much-welcomed resurgence this year here, advancing to her first Grand Slam quarterfinal since the 2014 US Open, when she went on to the finals and lost to Serena Williams.

Don’t get me wrong. It’s great to see Keys or any American player break into the top 10. Her eighth seed is Keys’ highest at a Grand Slam. She’s reached the round of 16 or better at seven of her last eight Grand Slams. But I look forward to seeing her improving, adding some variety to her game and cutting down on unforced errors.

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Keys has had a variety of coaches, including Jesse Levine, Lindsay Davenport and Jon Leach. Her current coach is Thomas Hogstedt. She hits the ball with much power. And she has talent. Now she has to learn to harness that power game and play smart.

“I think it was nerves a little bit,” Keys said. “I definitely felt like I got off to a bad start, and then I felt like I was trying to catch up from there. I definitely don’t think I was playing my best. I mean, I think she played really, really well today. I think it was just a combination of me not playing my best and not playing super smart and her playing really well.”

Jo-Wilfried Tsonga, the ninth seed from France, stopped Socks’ impressive run here, 6-3, 6-3, 6-7, 6-2.

Sock has also enjoyed a breakout performance getting to Sunday’s fouth round, just his second fourth round in a major. He ousted 2014 US Open champ Marin Cilic to get there.

Against Tsonga, Sock got the crowd in a frenzy in the third set when he was able to win the tiebreaker, 9-7. Sock then left the court for a bathroom break, came back and wasn’t the same. Tsonga dominated the fourth set 6-2 to win the match.

Not to put any more pressure on the young Americans, but the time is now to learn from this “major” experience and make the necessary improvements to reach full potential. Let’s hope they both continue to improve and impress.

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Follow Ann on Twitter at @AnnLiguori