Wozniacki served for the match at 5-4 in the second set, but couldn't create closure and twice took treatment for a sore left wrist in the final set as the match slipped from her grip. Radwanska reeled off five straight games to spark a 3-6, 7-5, 6-2 victory that vaulted her into the Sydney semifinals and could cost Wozniacki her world No. 1 ranking.
If second-ranked Petra Kvitova, who beat Wozniacki in last week's Hopman Cup, takes the title in Sydney, she will surpass the 21-year-old Dane and seize the top spot Wozniacki has held for the past two years, save for a one-week stint Kim Clijsters spent at No. 1 last February.
A resourceful Radwanska held a one-break lead twice in the opening set only to see Wozniacki, who roared back from a 0-4 third-set hole against Dominika Cibulkova in the prior round, battle back.
Neither woman's serve evokes memories of an imposing Brenda Schultz-McCarthy, and both are accurate returners. The result was five breaks in an opening set offering extensive rallies as both players worked for the positional edge. The returner won 10 consecutive points during one stretch, as the pair traded love breaks before Wozniacki erased a break point and scraped out a hard-fought hold for 4-3. Radwanska then fought off five break points before successive backhand errors gave Wozniacki her third break and a 5-3 lead. Radwanska, who squandered leads at 1-0 and 3-2, stared down at her shoelaces in disappointment, committing 20 errors compared to Wozniacki's 10 in the 47-minute first set.
The down-the-line backhand is Wozniacki's kill shot, and she used it effectively to wrong foot her friend. But Wozniacki is not nearly as comfortable firing her forehand down the line, something Radwanska knew as she anticipated the cross-court forehand. In the second set, she began to gain traction in the rallies.
Wozniacki dumped a double fault into net—a timid end to the fourth game—and was soon facing a 1-4 hole before incredibly bursting back to take a 5-4 advantage. The Dane had the momentum, but Radwanska, who won 28 of 37 trips to net, used her soft hands and forward thinking to break back for 5-5. Wozniacki withstood four set points in the 12th game before successive backhand errors gifted Radwanska the set.
An emboldened Radwanska ran off eight straight points to take a 2-0 lead in the decider. Wozniacki broke back for 1-2 and spent the ensuing changeover getting her strained left wrist re-taped before holding for 2-all. It was her last stand. The seventh seed slapped an inside-out forehand that skimmed the top of the tape and plopped inside the sideline to break for 5-2, followed by a rare hold to seal the contest.
Radwanska snapped a streak of five losses to Wozniacki in beating her for the first time since 2007 to set up a semifinal with Victoria Azarenka, who has won five of their eight prior meetings. Kvitova faces defending champ Li Na in the other semifinal.
—Richard Pagliaro