With an eye on the Australian Open later in the month, Serena Williams worked her way into a blistering performance against Camila Giorgi.
Serena Williams will face Italian world number 52 Camila Giorgi on Tuesday for a place in the semi-finals.
Giorgi, ranked 136th in the world, had not played since a third-round run at Wimbledon because of a right shoulder injury. Wozniacki was caught off guard and quickly knew she was in trouble as Giorgi fired 46 winners in the match to only 13 for the Danish standout.
The 21-year-old Camila Giorgi, who won three matches in qualifying to earn her spot in the main draw, is the first woman qualifier to reach the fourth round of the U.S. Open since Anna-Lena Groenefeld in 2008.