Indeed, Azarenka wasn't much better, with nine double-faults of her own. Unable to serve out the match at 5-3 in the third set, Azarenka finally clinched the victory on her fourth match point.
On Wednesday, she'll face unseeded Daniela Hantuchova, who played her fourth-round match Monday.
The meeting between Azarenka and Ivanovic was the only one of four women's matches that day to be postponed to Tuesday because of rain. Asked if anything could have been done differently to keep everybody on the same schedule, Azarenka wouldn't bite. She just smiled and said, "Mother Nature could have done something.''
A few hours after Azarenka made the quarter-finals, the first woman reached the semis. Fifth-seeded Li Na beat No 24-seeded Ekaterina Makarova 6-4, 6-7 (5), 6-2 in 2 hours, 20 minutes.
Li, the 2011 French Open winner, was up a break three times in the second set, then led 3-0 in the tiebreaker, but couldn't close the victory. From 2-2 in the third, she won four straight games to reach her first US Open semifinal.
Azarenka, the two-time Australian Open champion insisted she's not concerned by two straight slow starts. She also lost the first set in the third round to 26th-seeded Alize Cornet before rallying for a 6-7 (2), 6-3, 6-2 win.
She's gotten into it with the chair umpire in two straight matches as well. On Tuesday, it was over Ivanovic being awarded a winner, instead of the two replaying the point, after a delayed out call was overturned in the first set.
Azarenka looked sluggish after a late-morning start and didn't make her first winner until the sixth game. But Ivanovic has often wilted under pressure since her lone Grand Slam title more than five years ago; she hasn't been back to a major semifinal since.
Azarenka started winning points on drops shots she said she's always had that in her repertoire but just hadn't need them much before.
There were five straight breaks to open the second set. After Ivanovic double-faulted on break point for the second straight service game, Azarenka finally held to lead 4-2.
The start of the third set was much of the same. Azarenka held in the first game, then there were four straight breaks.
Azarenka preferred to joke about the two players' serving struggles: "I think we're just great returners.''
Azarenka had two match points while serving at 5-3 but wasted both with backhand errors. The way Ivanovic was serving, though, Azarenka was still in prime position to close the match, and sure enough, she broke Ivanovic again in the next game to advance.
It took 2 hours, 10 minutes, but the 2012 US Open runner-up wasn't going to fret about her uneven play.
"If I have to fight and dig deep to find a way not playing at your best against a player who was playing that good,'' Azarenka said, "I think it's pretty remarkable to walk off the court with a win.''
-AP
