Defending champion Serena Williams cruised into the quarter-finals of the Western & Southern Open with a 6-0 6-2 win over Italy's Karin Knapp on Thursday while Swiss teenager Belinda Bencic's remarkable run came to an end.

Williams, who was upset by Bencic last week in the semi-finals of the Rogers Cup in Toronto, needed 54 minutes to get by Knapp and set up a showdown with Serbian Ana Ivanovic in what will be a rematch of last year's final.

"When you've done well somewhere it gives you confidence and also takes pressure off," said Williams, who has won 41 straight matches on U.S. hardcourts since losing to Victoria Azarenka in the 2013 Cincinnati final.

"Playing at home in America is great."

Sixth seed Ivanovic advanced after beating American Sloane 2-6 6-4 6-1. The Serb has beaten Williams only once in nine meetings with the only win coming at in a last-16 clash at the 2014 Australian Open.

Rogers Cup champion Bencic took an eight-match winning streak into her last-16 clash with Lucie Safarova but the 18-year-old Swiss was forced to retire with a muscle strain after her seventh-seed opponents took the first set 6-2.

"First of all, I don't think it's very serious," Bencic told reporters. "I just wasn't 100 percent today, and to beat Lucie or even compete against her, you need to be 100 percent."

Bencic will meet with her team before deciding whether she can play in the Connecticut Open next week, the final tournament before Aug. 31-Sept. 13 US Open.

Bencic began her match against Safarova by breaking her opponent, but Safarova broke right back to win with ease and will next face either Ukrainian 14th seed Elina Svitolina or Frenchwoman Caroline Garcia in the quarter-finals.

Unseeded Russian Anastasia Pavlyuchenkova also advanced to the quarter-finals when Azarenka retired despite leading the match 6-1 0-3.

Serbian Jelena Jankovic and Slovak qualifier Anna Karolina Schmiedlova also advanced to the quarter-finals.