It’s no secret that Serena Williams’ main goal over the next few weeks is to sharpen up her game for the U.S. Open. In that enterprise, she faced an interesting opponent in the second round of the Rogers Cup in Montreal today—the woman who stunned Williams in the 2011 U.S. Open final, Sam Stosur.

Granted, this is no longer the same Sam Stosur who played an aggressive, athletic game long on precision and finesse to win that match. She’s won three matches in succession just twice this year (at Hobart, back in January, and then at the French Open two months ago), and last week in the first round at Stanford was out-dueled by Japan’s Naomi Osaka, whose world ranking at the time was No. 406.

Stosur didn’t seem like much of a threat, but Williams treated her as if she were. Perhaps she wanted to remind her of that September night in 2011, when Stosur took the food right off Williams’ plate. Whatever the case, Williams played her best match in months, crushing Stosur in just 58 minutes, 6-0, 6-2.

The beginning had to be particularly painful for Stosur fans to watch. Williams ripped through the first four games in barely 10 minutes. Because of the power blackout still effecting the Stade Uniprix, even the live scoring module at the WTA website was down. It hardly mattered.

At 0-4, Stosur still had a chance to perhaps get back in it, given how loosely and liberally Williams had donated games to rivals in recent matches. But this was a different Serena Williams, one who would not allow her opponent a break point in the entire match. Stosur fell behind 15-40 in that fifth game and never recovered; an ugly inside-out backhand error to give Williams her third break of the set.

Williams wasted no time serving it out; from 30-15 she drilled an ace down the T and then watched Stosur belt a backhand way over the baseline. In that first set, Williams hit four aces and won nine of the ten first serves she put into play. She won three of the four second serves she was obliged to hit. Stosur won a grand total of seven points. You might wonder where Williams found the time to hit all those serves in a set that lasted all of 18 minutes.

When a player so dominates a set, a letdown is almost a given. And in some cases, emerging from that magical zone is disconcerting enough to turn a blowout into a match. Stosur stopped the bleeding and eked out a hold to start the second set (fending off a break point along the way), thanks to a backhand service return error by Williams. As Williams served out the next game to hold for 1-1, rain drops began to pitter-patter on the court. The women remained in their chairs for about a minute and then were sent off at 1-1.

They returned after a 30-minute rain delay, and soon Stosur got to 30-all in the third game. But she wasn’t able to stem the tide. She hit drove a tentative backhand into the net off a Williams service return to give Serena a break point. She survived that one, but not the next: Williams took the game with a cross-court forehand winner that put her up, 2-1.

Williams fell behind 15-30 in the next game, but she recovered to hold. Stosur then appeared to be broken again, but she challenged a break-point call in Williams’s favor and was proven right. She went on to hold and stay within one break at 2-3. But her fingernails were scraping the face of the cliff. Williams held for 4-2, then broke Stosur again with ease. Leading 5-2, Williams built a 40-15 lead and served just one more ball—a match-ending ace.