Quarterfinal play for the Rogers Cup opened Friday, with a number of marquee stars fighting for semifinal berths in Canada.
Fans in Toronto got a rematch of the 2018 U.S. Open final as the eighth-seeded Serena Williams met the second-seeded Naomi Osaka in the day's biggest matchup.
Both defending champions remain alive. Simona Halep opposed Marie Bouzkova, while Rafael Nadal took on Fabio Fognini.
Here's a recap of Friday's action.
2019 Rogers Cup Results, Aug. 9
 Rafael Nadal def.  Fabio Fognini 2-6, 6-1, 6-2
 Daniil Medvedev def.  Dominic Thiem 6-3, 6-1
 Karen Khachanov def.  Alexander Zverev 6-3, 6-3
 Serena Williams def.  Naomi Osaka 6-3, 6-4
Bianca Andreescu def.  Karolina Pliskova 6-0, 2-6, 6-4
Marie Bouzkova def.  Simona Halep 6-4(ret.)
Sofia Kenin def.  Elina Svitolina 7-6(2), 6-4
Williams avoided a repeat of the U.S. Open final, taking down Osaka in straight sets.
The 23-time Grand Slam champion was nearly untouchable on serve. She didn't allow a single break-point opportunity and collected 12 aces with only three double-faults, according to the Rogers Cup's official site.
Osaka saved two break points in the sixth game of the first set but wasn't so lucky two games later, which was the decisive blow Williams needed to take the opening frame. A service break in the third game of the second set helped her secure the victory.
In addition to her overpowering serve, Williams' backhand was a reliable weapon all night:
This was Williams' first win over Osaka, having dropped their first two meetings. Osaka took the defeat in stride:
Learned so much in the past couple of days. Played the g.o.a.t 🐐 , came back to #1 AND most importantly started smiling again during matches. I’m so happy honestly 😊 can’t wait to play more matches and learn from life. Thank you Toronto see you next time 🤗 https://t.co/YpX13VnW5o
Halep's night ended early. After dropping the opening set 4-6, she was briefly evaluated by trainers, and it was determined she could no longer continue:
Following her three-set win over Jennifer Brady in the round of 32, Halep revealed she had experienced trouble with her Achilles.
"I was tired in the third set," she said, per Alex Macpherson of the WTA's official site. "I felt the Achilles since the first point of the match. But it's normal after a break and also coming on the hard courts. It's very difficult for the ankles and also for the knees. But I'm happy to go through, and hopefully tomorrow I will be OK."
Although Halep went on to beat Svetlana Kuznetsova in the third round, her Achilles clearly flared up again and prevented her from successfully defending her Rogers Cup crown.
Nadal fared better, though not without some difficulty. He dropped serve twice in the first set to fall behind to Fognini before regrouping and steamrolling the 32-year-old Italian over the remainder of the match.
"I lost [the first set] 6-2 with the feeling I was not playing bad, no negative feelings on the ball, just things [were] going too fast," Nadal said after the match, per the ATP's official site. "Honestly, the beginning of the match [was] difficult because I [was] playing probably a little bit better than him, but then the score was against me. I felt that I had a big chance to be 3-Love for me. Then it was 3-1 for him."
Fognini and Nadal combined for the best point of the tournament. The crowd's reaction grew as the action unfolded, reaching a crescendo at Fognini's drop shot:
Nadal is now through to the semifinals for the ninth time in 10 tournaments.
 Karen Khachanov vs.  Daniil Medvedev (not before 3 p.m. ET)
 Rafael Nadal vs.  Gael Monfils/ Roberto Bautista Agut (not before 8 p.m. ET)
Sofia Kenin vs. Bianca Andreescu (1 p.m. ET)
Marie Bouzkova vs.  Serena Williams (not before 6 p.m. ET)