It was a rough 15 minutes for the crowd favorite, who five times stood two points from claiming the set, but couldn't get closer. When Roddick served for it at 5-3, he played a loose game, rolling his eyes after putting one backhand into the net, then sailing an approach shot long and rushing a forehand long. Del Potro broke there and eventually, as drops began to fall, they headed to the tiebreaker.
After only one point, an inside-out forehand winner by Roddick, chair umpire Carlos Bernardes stepped down to inspect the wet court and declared it unplayable. A few spectators booed. Roddick and del Potro sat in their changeover chairs for a few minutes, until being told the delay would be substantial enough that they could wait it out in the locker room.
Not much after that, they found out they would need to come back Wednesday, when the forecast calls for an 80 percent chance of rain.
Azarenka and Stosur needed to wait out a 75-minute delay in their first set, but at least they got done.
Stosur never had taken so much as a set off Azarenka in six previous tour meetings, including one match in qualifying. That changed in Tuesday's second set, although forcing a third might not have given Stosur all that much self-belief. And in the end, Azarenka improved to 11-0 in three-setters this season, while Stosur fell to 9-7.
"I think I'm capable of beating her one day," the seventh-seeded Stosur said. "Just would have liked it to have been today."
Serving at 5-all in the third, Azarenka faced a break point and responded the best way possible, delivering a 92 mph ace, her only one of the match.
Asked about that at her news conference, Azarenka's response was telling: "When did I hit an ace? Did I hit one today, actually?"
Her serve is not exactly her strongest stroke, and Azarenka double-faulted five times, including while ahead 5-3 in the tiebreaker.
Azarenka used a swear word at her news conference to describe the second fault there, which landed several feet long. That was part of a topsy-turvy tiebreaker, in which Azarenka went ahead 4-0, and Stosur took five of the next six points to make it 5-5.
That's when Azarenka buckled down.
"For sure, she really pushed me to dig deep," Azarenka said.
On the next point, a Stosur groundstroke clipped the net and went over, and Azarenka had the presence of mind, and soft hands, to respond with a drop-shot winner. That set up match point, which fittingly was a hit-'em-hard, 10-stroke exchange, until Stosur sailed a backhand long.
Azarenka dropped her racket, put her hands to her face, then looked skyward and said, "Oh, my God."
She went to the middle of the court and raised her right index finger overhead — yes, she's No. 1 in the rankings, and she'll stay there for the time being — before blowing kisses to the crowd and throwing a ball into the stands.
She said later she was unaware that her victory locked up that top spot, and explained that a U.S. Open trophy would mean a lot more.
Follow Howard Fendrich on Twitter at http://twitter.com/HowardFendrich
Copyright 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.