Rhythmically looping her service motion during the opening coin toss, Serena Williams spent much of the match swinging with aggression against an unbreakable opponent.

In a tense serving test devoid of a single break, Williams held her nerve and guarded her serve with fervor in fighting off Stosur, 7-6 (7), 7-6 (7), to reach the round of 16 in Cincinnati. The world No. 1 competed with resilience, unleashed some gutsy 100 M.P.H. second serves at crunch time, fended off two set points in the second-set breaker, and exploited a couple of key misses from the Aussie to prevail in a two-set thriller.

Williams had won seven of 10 prior encounters with Stosur, including a 6-0, 6-2 shellacking in Montreal last week. There's a certain sense of liberation that can come from being blown out: No matter how bad things get, they probably won't get that ghastly again. Playing with purpose and stinging Serena with her forehand down the line, Stosur showed a strong sense of belief from the start.

Burying a backhand into the bottom of the net, Williams faced her first—and only— break point of the day in the third game. A 110 M.P.H. dart down the middle, punctuated by a firm "Come on!" dismissed it. Dashing to her right, Williams wrong-footed Stosur curling a cross-court forehand, eventually holding for 2-1.

When Stosur's first serve is firing right, it ignites the rest of her game. Issuing a two-ace game to hold for 4-all, Stosur repeatedly jammed the top seed with a twisting body serve and dotted all areas of the service box, permitting just four points through her first four service games.

Serving at 5-6, Stosur saved a set point by firing a forehand down the line, ultimately forcing the tiebreaker. She then took advantage of a Williams double fault for a 4-0 lead. At that point, Williams' trainer, Esther Lee, leaned forward in her front-row seat with hands clasped as if praying for a comeback.

Down 3-4, Williams snapped off a sharp-angled forehand pass cross-court to level. Stosur drew a backhand error to save a second set point, then caught the back of the baseline to deny a third after Serena's unsuccessful challenge. On her fourth set point, Williams reached back for a 101 M.P.H. second serve down the middle to seal the 58-minute opener.

The 30-year-old Aussie caught Williams leaning right and smacked an inside-out forehand winner followed by a flat forehand down the line, digging thorough a demanding deuce game to hold for 3-2 in the second. New coach Simon Rea stressed the positives— "[You're hitting] with courage and aggression on the second serve, terrific"—and urged his charge to attack Williams' second serves with more aggression, saying, "Let's sow the seeds of doubt."

Dialing in her serve, Williams, who won 15 of 16 points played on her first serve in the second set, played with more conviction. She delivered three love holds in her last five service games to force another tiebreaker after one hour and 45 minutes of drama.

Williams had trouble reading Stosur's inside-out forehand at times; she used that shot to set up a forehand drive down the line for 5-4, then gained double set point. A skittish Stosur blinked. She netted a rally forehand on the first set point, then spun a forehand long on the second.

Williams' second serve is a confidence shot, and it was the keystroke today as she attacked it with vigor when it mattered most. Sliding a 100 M.P.H. second serve wide, Williams twirled in celebration when Stosur's return missed the mark for match point. A Stosur backhand expired in net, ending a quality one hour, 55-minute battle. "Thanks Sam," Williams said at net. "Good playing," Stosur replied.

The top seed will face either 13th-seeded Italian Flavia Pennetta or 18-year-old Taylor Townsend for a quarterfinal spot.