NEW YORK (AP) When the U.S. Open decided to give 2009 champion Juan Martin del Potro a wild-card invitation this year, it rankled American player Steve Johnson.
What if, Johnson wondered aloud, del Potro was to wind up beating someone from the United States in the first round of the country's Grand Slam tournament?
That didn't come to pass: del Potro faced a fellow Argentine in his opening match at Flushing Meadows. As it happens, though, del Potro's opponent in the second round in Arthur Ashe Stadium on Thursday night will be - yes, you guessed it - Johnson himself.
As it is, Johnson barely made it to this intriguing matchup.
In the first round on Tuesday, the 19th-seeded Johnson, who won two NCAA singles titles at Southern California, dropped the opening two sets and needed to save six match points before coming all the way back to beat Evgeny Donskoy 4-6, 1-6, 7-6 (2), 6-3, 6-3. Johnson saved four match points while trailing 5-2 in the third set, then two more while down 6-5 in that set.
Afterward, Johnson acknowledged that he came out tight in that match because he felt the pressure of being an American playing in the U.S. Open.
''I shouldn't let the moment get the best of me,'' he said, ''and it did for a bit.''
Del Potro had a much easier time in his opening match, beating countryman Diego Schwartzman 6-4, 6-4, 7-6 (3).
A little more than two weeks ago, fresh off earning a silver medal at the Rio de Janeiro Olympics, del Potro was awarded his wild card, allowing him into the main draw of the U.S. Open for the first time since 2013. He missed 2 1/2 years' worth of major tournaments because of three operations on his left wrist, only returning to Grand Slam action at Wimbledon in June.
Once ranked as high as No. 4, he is currently at No. 142 because of so much time away from the tour, and he wasn't high enough to gain direct entry into the field at Flushing Meadows.
''The worst part of my life is totally in the past,'' del Potro said, ''and I'm living a good present and looking forward for a good future.''
Here is what else to watch on Day 4 of the U.S. Open:
WILLIAMS SISTERS: Both Williams sisters will be in action, in consecutive matches on Ashe. First, No. 6-seeded Venus Williams will take on Julia Goerges of Germany in the last day session match in the main stadium. Then No. 1 Serena Williams will face Vania King in an all-American matchup to start the night session. The siblings are coming off quite contrasting first-round outings on Tuesday. Venus labored during a three-set victory. Serena, who came to the U.S. Open with questions about a sore right shoulder, looked perfectly fine during her straight-set win, pounding serves at up to 121 mph, including 12 aces. Serena is bidding for a 23rd Grand Slam singles title - that would break a tie with Steffi Graf for the most in the Open era, which dates to 1968.
MURRAY: Three-time major champion and two-time Olympic singles gold medalist Andy Murray faces Marcel Granollers in the afternoon. They've played seven previous times, with Murray winning six. Murray, who won his first Grand Slam title at the U.S. Open in 2012, can become only the fourth man in the Open era to reach the final of each major tournament in a season, joining Novak Djokovic, Roger Federer and Rod Laver. Murray lost to Djokovic at the Australian Open in January, and the French Open in June, then won Wimbledon in July.
FORECAST: There is rain predicted for Thursday, which used to create major scheduling headaches at the U.S. Open, but shouldn't be as much of a problem now. That's because the tournament finally has a retractable roof atop Ashe - and the $150 million structure passed its first test, used during a match for the first time Wednesday night as Rafael Nadal beat Andreas Seppi.
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