The 37-year-old Williams has won the event three times, all in Toronto, and has a 31-4 match record.

Serena Williams won her first match since losing the Wimbledon final, beating Elise Mertens of Belgium 6-3, 6-3 on Wednesday night in the second round of the Rogers Cup.

The 37-year-old Williams has won the event three times, all in Toronto, and has a 31-4 match record.

"I feel like my movement is great — been working on my fitness, so I felt like it really was able to shine through today," Williams said. "I'm loving going out there, and I'm loving kind of running. So it's a good thing."

In the third round, she'll face Russia's Ekaterina Alexandrova — a 6-4, 6-3 winner over Zhang Shuai of China.

"Now that I'm just injury-free, I'm just enjoying being able to train, and I haven't been able to do it since January, really," said Williams, seeded eighth. "So I just think that the fact that I can train and practice and get in the gym is really going to be helpful for me."

Wimbledon champion Simona Halep of Romania beat American qualifier Jennifer Brady 4-6, 7-5, 7-6 (5).

The winner last year in Montreal, Halep lost a 4-0 lead in the third set when Brady won five straight games. Halep broke Brady for the third time in the set to go up 6-5, but Brady broke back to force the tiebreaker.

"It was a very, very tough one," Halep said. "I expected it because I knew that she's going to serve big and also the forehand is big. I didn't feel 100% ready for the tournament because I had a long break (after Wimbledon). But I'm really pleased with the way it was today, the fact that I fought till the end."

The 26-year-old Halep will face Svetlana Kuznetsova of Russia. Kuznetsova beat Donna Vekic of Croatia 7-6 (4), 6-3.

Second-ranked Naomi Osaka of Japan reached the third round when German qualifier retired because of a left abdominal injury. Osaka led 6-2 in her first match since losing in the first round at Wimbledon.

Canada's Bianca Andreescu beat Russia's Daria Kasatkina 5-7, 6-2, 7-5. On Tuesday night, the 19-year-old Andreescu beat fellow Canadian Eugenie Bouchard.

Third-ranked Karolina Pliskova of the Czech Republic also advanced, beating American qualifier Alison Riske 6-4, 6-7 (4), 6-2

No. 16 seed Anett Kontaveit of Estonia reached the third round when Carla Suarez Navarro of Spain retired with a hip injury while trailing 7-5, 3-1. Kontaveit will face Pliskova on Thursday.

Victoria Azarenka of Belarus was eliminated, losing 7-5, 7-5 to Dayana Yastremska of Ukraine.

Ukraine's Elina Svitolina, the 2017 Rogers Cup winner, moved onto the third round with a 6-3, 3-6, 6-3 victory over Katerina Siniakova, and 2015 champion Belinda Bencic of Switzerland edged Julia Goerges 5-7, 6-3, 6-4. Bencic and Svitolina will face each other Thursday.