"I was really frustrated with how I was playing," Williams said. "And then I got really angry. ... So I said, 'Serena, you're going to have to be positive and be good to yourself out here.' "
The 33-year-old Williams blamed unforced errors for her early struggles.
"It was a good opportunity for me because I haven't played a match on hard courts since April-ish, early, early April, after Miami," Williams said. "It was a long time ago. So I think it was really good for me to have that match play."
Williams won first of her three Toronto titles in 2001.
"Like a nice vintage wine," Williams said. "A good vintage red wine, I'd say, getting better with age, I hope."
Also, France's Alize Cornet upset ninth-seeded Carla Suarez-Navarro of Spain 6-3, 6-7 (2), 6-4, qualifier Mirjana Lucic-Baroni of Croatia beat 10th-seeded Karolina Pliskova 3-6, 7-6(5), 6-2, and No. 11 Ekaterina Makarova of Russia topped American qualifier Anna Tatishvili 6-3, 6-3