It's rare for a player to win consecutive matches this deep in a Slam after losing both 1st sets unless they're a multiple Slam winner. I wouldn't bet Halep, chances are strong she runs out of gas even if she forces a 3rd.
The match was decided in the 1st set. Halep was lehtal off the ground against Kerber's serve, but pretty much all at sea on her own serve. Inexplicable. If she was poor in both instances, it would've been understandable. But the sheer fact of serving sent her ground game awol.