Williams' winning streak is up to 17 matches following titles in Charleston, S.C., and Madrid. She also has two victories in Fed Cup.
Pennetta called for the trainer after falling behind 3-0. Her wrist was retaped but the Italian conceded during the fifth game after 28 minutes of play.
"I felt pain during the second or third point of the opening game," Pennetta said. "Something happened on the backhand. ... Then not being able to serve and attack like usual, there wasn't much else to do."
For Serena, who won this tournament 10 years ago, the quick match was welcome.
"I have played every day and I haven't had a day off," she said.
Kvitova has an abdominal injury and needed to be bandaged again after losing the first set. That seemed to work. She stormed through the second set, easily winning the first five games.
Kerber, however, came back in the final set and took the match when Kvitova netted a ball. The victory put Kerber into her sixth semifinal of the year and into the top 10 in the rankings for the first time in her career.
Information from The Associated Press was used in this report.