We consider an underlay relay-assisted two-hop cognitive link which employs best relay selection to enhance the performance of secondary transmissions while keeping the interference at the primary receiver below a specified level. Best relay selection is implemented using countdown timers without cooperation between the relays. Assuming channel knowledge at the relays, the best relay is determined by the maximum signal-to-noise ratio (SNR) at the secondary receiver. Due to the timer-based operation, collisions may occur and the timer design is optimized to minimize the collision probability. The performances of the amplify-and-forward and decode-and-forward modes of operation are compared. Closed form expressions for the secondary outage probability are derived for both schemes. Numerical results provide insight into the system operation and how the outage probability depends on the various system parameters. © 2014 IEEE.