If you try to install Hyper-V inside of VMware Workstation you will most likely get the error message “Hyper-V cannot be installed: A hypervisor is already running”.

To get around this error, power down your VM. Then, within VMware Workstation, click VM->Settings. From the Virtual Machine Settings page, select the options tab. Under the General settings on that tab you’ll notice the Guest Operating System area on the right half of the dialog screen. Choose “Hyper-V (unsupported)” as the operating system version.

After that change, it still didn’t work for me. I powered down and then enabled the “Virtualize Intel VT-x/EPT or AMD-V/RVI” setting on the Processors item.

After making that change I was able to complete the installation and use Hyper-V on my Windows Server 2012 R2 system running inside VMware Workstation.

I hope this saves you some time in troubleshooting this yourself!