By Larry Fine

NEW YORK (Reuters) - Serena Williams battered fourth-seeded Victoria Azarenka in the first set and then prevailed in a dramatic tie-break to close out a 6-1 7-6 win and move into the round of 16 at the U.S. Open Saturday.

Williams, seeded 28th after a long layoff due to injury and health concerns, dropped her from the world number one ranking, looked set to turn a tough match-up into a rout before the Belarussian raised her game and battled the three-time champion on even-footing in the second set.

World number five Azarenka fought off four match points in a second-set struggle that produced the best tennis of the women's tournament so far to force a tiebreaker.

The decider was tied at 5-5 when Azarenka, a semi-finalist at Wimbledon this year, netted a forehand to give Williams her fifth match point.

Williams did not squander this one, inducing Azarenka to send a forehand wide to claim the tie-break 7-5.

The 29-year-old Williams will next face either former French Open champion Ana Ivanovic, the 16th seed, or American wildcard Sloane Stephens.

(Editing by Julian Linden)