Serena Williams reacts after winning a point against Kiki Bertens, of the Netherlands, during the second round of the U.S. Open tennis tournament, Wednesday, Sept. 2, 2015, in New York. (AP Photo/Charles Krupa)

NEW YORK (AP) — Serena Williams’ bid for a true Grand Slam is safe, despite an uneven performance in the U.S. Open’s second round.

Overcoming 10 double-faults and two dozen other unforced errors, Williams emerged with a 7-6 (5), 6-3 victory over 110th-ranked qualifier Kiki Bertens of the Netherlands at Arthur Ashe Stadium on Wednesday.

Williams got broken to fall behind 2-1 early, and Bertens served for the opening set at 5-4 before getting broken for the first time.

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 has won the past four major titles, a streak that began at last year’s U.S. Open, and is trying to complete the first calendar-year Grand Slam since Steffi Graf in 1988.