Stosur, the US Open champion, was coming off her first win over Maria Sharapova late Tuesday but appeared flat as she came out for Wednesday’s opening match at the season-ending event among the top eight players.
In her first match in the White Group, the fourth-seeded Azarenka broke four times to improve her record against Stosur to 5-0.
Later, French Open champion Li Na defeated Sharapova 7-6 (4), 6-4, while top-ranked Caroline Wozniacki played Vera Zvonareva of Russia in the day’s final match.
Tipsarevic advances in St. Petersburg
In St. Petersburg, Russia, second-seeded Janko Tipsarevic of Serbia advanced to the second round of the St. Petersburg Open with a straight-set victory on Wednesday, while sixth-seeded Marcel Granollers of Spain was beaten.
Tipsarevic beat Olivier Rochus of Belgium 6-2, 7-5. Igor Andreev of Russia rallied to oust Granollers 3-6, 6-4, 7-6 (4).
Later, fifth-seeded Mikhail Youzhny of Russia advanced to the quarterfinals, beating Julien Benneteau of France 6-4, 3-6, 6-2 in their second-round match.
Fresh from winning his second career title in Moscow on Sunday, Tipsarevic broke Rochus twice in the first set and served for the match at 5-2 up in the second. But Rochus, ranked 70th, broke him twice to level at 5-all.
The 13-ranked Serb then broke for the third time and served for the match. He double-faulted on his first match point at 40-30 up before closing the match on his second one.
Youzhny, who won the event in 2004 and was runner-up last season, broke twice in the deciding set for a 5-2 lead. He then saved a break point before wrapping up the victory on his second match point.
The 117th-ranked Andreev saved a match point while serving at 5-4 down in the decider to force a tiebreaker.
Serving at 5-4 up in the tiebreaker, Andreev landed a precise backhand down the line to earn two match points and then served a winner to close the match.
Andreev said he was struggling with his first serve in the first set.
“But it could have gone either way in the tiebreaker,” said Andreev, who lost in a third-set tiebreaker in the first round at the Kremlin Cup last week. “Last week I lost (in a tiebreaker). I decided to play more aggressively and risk today and I won.”
With the victory, Andreev, a former top-20 player, ended a five-match losing streak on hard courts.
“It was a very close match,” Granollers said. “We both played good but in the tiebreaker Igor (Andreev) was better than me. When you are in the tiebreaker, the match can be decided by two points.”
Also in the first round, Philipp Kohlschreiber of Germany rallied to beat Carlos Berlocq of Argentina 6-7 (4), 6-3, 6-1, while Andreas Seppi of Italy eliminated Vasek Pospisil of Canada 7-5, 7-6 (4).
Del Potro reaches quartefinals
In Vienna, Juan Martin Del Potro defeated Philipp Petzschner of Germany 6-3, 6-4 Wednesday to reach his seventh quarterfinal of the season at the Erste Bank Open.
The second-seeded Argentine, who is seeking one of three remaining spots at next month’s ATP finals, saved four break points in the opening set but held serve throughout against Petzschner, who won his only tour title here three years ago.
In first-round play, Steve Darcis of Belgium came back from 4-0 down in the deciding set to upset No. 7 Nikolay Davydenko of Russia 3-6, 6-0, 7-6 (2).
Also, Jarkko Nieminen of Finland defeated Robin Haase of the Netherlands 7-6 (3), 6-4 to set up a second-round match against top-seeded Frenchman Jo-Wilfried Tsonga.
