If there is a single, overriding lesson to this decade’s developments in American men’s tennis, it is to have a short memory.

“I mean, obviously, right now it’s great,” 19-year-old Jared Donaldson said after his wowsa upset of Belgium’s 12th-seeded David Goffin. “During the match, just briefly after, it was great to win. But, I mean, it’s kind of on to my next round.”

For Donaldson, it was easy to hurry along after what he called a “really, really special victory.” He is ranked 122nd and needed three wins in the U.S. Open qualifying tournament just to shoulder his way into the main draw before summarily dismissing Goffin, a French Open quarterfinalist this year, 4-6, 7-5, 6-4, 6-0.

But, all around Donaldson yesterday was the flip side of the need to forget, that familiar sense of U.S. men experiencing what victims at a carnival dunk tank know.

Sam Querrey, just two months after the personal highlight of shocking No. 1 Novak Djokovic at Wimbledon, was sent packing in four sets by Serbian veteran Janko Tipsarevic, who lately has recovered from serious illness and is ranked a humble 250th.

Rajeev Ram, a 32-year-old qualifier more accomplished at doubles, lost to 64th-ranked Brit Daniel Evans. Young hopefuls Michael Mmoh, 18, Denis Kudla, 24, and Christian Harrison, 22, all lost.

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That left only Steve Johnson and Donald Young to join Donaldson in feeling more like a hammer than a nail. Johnson, who earlier this month briefly slipped past John Isner to become the top-ranked American at No. 22, had to apply an in-match short memory to recover from a two-set deficit and edge 79th-ranked Evgeny Donskoy of Russia. Young, the 27-year-old former phenom whose many shifts and turns in an up-and-down career reflect general U.S. tennis vertigo, prevailed over No. 78 Jan-Lennard Struff of Germany in four sets.

“Every day can’t be perfect,” said Harrison in a succinct summary of tennis life, constantly swiveling one’s head to follow good vs. bad moments, invigorating success vs. depressing failure.

“At the end of the day,” said Querrey, who at 28 is ranked 30th after a career high of 17th five years ago, “tennis is great. I love it. But there’s other things in life, and you sort of separate it and move on. I was really bummed out for an hour right after the match. But, you know, kind of let it go and move on and get ready for playing doubles.”

Harrison, who missed more than two years of competition while undergoing seven operations — abdominal, right shoulder, left wrist, on and on — acknowledged the challenge of dealing with emotional whiplash. “Especially when it’s a week-to-week sport,” he said. “Because, unless you win the tournament, you’re going to lose. So you’ve got to get comfortable with that real quick.

“I think about not being able to play at all. I’d rather be losing in the first round of the main draw and feeling good about coming through the qualies. You always find ways to put it in perspective.

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“I look at my phone and the messages from my close friends that, ‘Hey, don’t get down. Keep your head up.’ And I’m, like, they’re right. That’s the way you have to handle it.”

The matches themselves are a microcosm of the process, Donaldson said, because “you run through so many scenarios in your head.” It’s a don’t-think-twice, it’s-all-right mindset.