NEW YORK (AP) — This was, in just about every measurable way, an up-and-down performance by Serena Williams, filled with double-faults — 10 in all — and two dozen other unforced errors.

With her bid for a true Grand Slam at stake, and facing a qualifier ranked only 110th, Williams was far from dominant in the second round of the U.S. Open on Wednesday, before pulling ahead and pulling out a 7-6 (5), 6-3 victory over Kiki Bertens of the Netherlands.

"Today, I was a little tight," Williams said in her on-court interview. "I think it showed."

Sure did.

She got broken early. She didn't manage to earn a break point against the strong-serving Bertens until the 10th game. Williams repeatedly was rather close to letting the opening set slip away. But as the 33-year-old American has demonstrated time and time again during her remarkable career, she can bring out her best when she needs it.

"I know that I can try to make a comeback," Williams said, "or try to make a run for it."

Williams trailed 2-1, and didn't hold a single break point until it was almost too late, with Bertens serving for the first set at 5-4.

Then in the tiebreaker, Williams trailed 4-0, before turning things around, helped by shaky play from Bertens, who only once has made it to the third round at a major tournament in 14 appearances.

Williams, in contrast, 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.

Perhaps Williams' uneven showing against Bertens was a result of being a tad rusty. In the first round on Monday, Williams' opponent, Vitalia Diatchenko, hurt her left foot while running sprints before the match and could barely move. Williams won 32 of 37 points in that one, which lasted about a half-hour until Diatchenko stopped playing while down 6-0, 2-0.

Bertens provided a sterner test, hitting serves at more than 110 mph and hanging in with Williams on lengthy baseline exchanges. Truth is, though, Williams was her own biggest problem, especially with all of those double-faults.

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