Kerber, from Germany wilted in the sticky heat at IGA Stadium in her first match since Wimbledon.
"It is one match, I know I have to learn from it," the fourth-seeded Kerber said. "I took a few weeks off after Wimbledon so I knew I had to come here, play a lot of matches, and also to get used to the hard courts again.
"Of course, it was not the plan to play just one match, but at the end, this is tennis. Every tournament starts from zero. Now I'll try to get ready for the next tournament."
Top-seeded Simona Halep of Romania trailed Anastasia Pavlyuchenkova of Russia 4-3 when rain forced the suspension of play for the day. Second-seeded Caroline Wozniacki of Denmark, and No. 13 Venus Williams never started their second-round matches.
Maria Sharapova continued her bid to climb back to the top of women's tennis with a 6-0, 6-2 victory over 12th-seeded Daria Kasatkina. Sharapova, the former world No. 1 who was ranked 149th a year ago after serving a 15-month doping ban, got a second one-sided win in a row.
"I've had all types of matches in my career," said Sharapova, now No. 22 in the world. "I'll take what I can get as long as I'm the one getting a chance to play the next match."
The Russian will face sixth-seeded Caroline Garcia of France in the third round. Sharapova is 4-1 in previous meetings, but Garcia won their last one this year in Stuttgart.
The top eight seeds had gotten a bye to the second round.
Defending champion Elina Svitolina of Ukraine, seeded fifth, led Mihaela Buzarnescu 6-3, 6-7 (5), 4-3 when the Romanian retired because of an ankle injury. Buzarnescu won last week in San Jose for her first tour title.