PARIS — Kiki Bertens stood a few feet behind the baseline and watched Serena Williams carve out the underside of a tennis ball and drop it gently over the net where it settled into Berten’s side of the court.
It was early in the first set, and Bertens had been matching Williams, the defending champion and top player in the world, point by point in their semifinal. But Bertens was slowed by a strained left calf suffered the day before, and Williams keyed in on it with merciless intent.
Then again, Williams did not look particularly mobile, either.