NEW YORK — The enormity of what is at stake for Serena Williams at the U.S. Open — the first true Grand Slam in tennis in more than a quarter of a century — hit her Wednesday.

That, she said, is why her play was so uneven in the second round at Flushing Meadows, despite facing a qualifier ranked only 110th.

And it’s why, after the 10 double-faults, two dozen other unforced errors and an all-around sloppy first set, Williams got pointers from coach Patrick Mouratoglou and headed straight to a practice court to put in work, hoping to repair what plagued her during a 7-6 (5), 6-3 victory over Kiki Bertens of the Netherlands.

“Today, I was a little tight,” Williams said. “I think it showed.”

Sure did. She got broken early. She double-faulted four times — yes, four — in one game. She didn’t manage to earn a break point against the strong-serving Bertens until the 10th game. Williams trailed 5-3 in the first set, finally broke for 5-all, but then needed to erase a 4-0 deficit in the tiebreaker.

All attributable, at least in part, to thinking about what she is trying to accomplish these two weeks.

“Until today, I was OK with it. I just got a little nervous today,” the 33-year-old American said. “But I’ve been doing totally fine. I’ve been completely relaxed, chill. I’ve been really, really fine. So I’m going to get back into the place that I was, and I’ll be fine again.”

She has won the past four major titles, a streak that began at last year’s U.S. Open, and 21 overall. If she can win five more matches at Flushing Meadows — starting in the third round against Bethanie Mattek-Sands in an all-U.S. matchup Friday — Williams would complete the first calendar-year Grand Slam in tennis since Steffi Graf in 1988.