The former Hannover goalkeeper joined the Foxes last month.
"I'm really excited," Zieler told the Leicester Mercury.
"It's nice to be here after a long break and I'm looking forward to starting back. Today was our first day of testing – not everyone is back yet, but I've met some of the players and a lot of the staff and I'm happy to be here now.
"We have a good pre season schedule. We're travelling quite a bit, so now it's time to get prepared for that. We have a few weeks now to prepare for the start of the season."