A method, system and computer program product for dynamically establishing a client's Time Sharing Option (TSO) and/or Interactive Productivity System Facility (ISPF) session and maintaining the client's TSO/ISPF session on a zero-downtime/operating system (z/OS) host over multiple service calls. A session provisioning and maintenance (SPM) utility determines whether a TSO/ISPF session exists for a client making a request for a TSO/ISPF service. When a TSO/ISPF session does not exist for the client, session establishment is initiated by generating a TSO initialization module in a new UNIX System Services (USS) process. Session establishment involves initializing a user's own TSO address space through USS and using USS service call API's to make TSO/ISPF service call requests in the TSO address space. The SPM utility maintains the client's TSO/ISPF session over multiple service calls, bypassing the overhead of having to establish a new session for every service request which requires TSO/ISPF services.