Excerpts from abc7chicago.com:

A Chicago firefighter was injured Saturday morning while working a blaze at a high rise construction site downtown. They were taken to Northwestern Memorial Hospital in good condition. The fire department responded at 5:54 a.m. for a call of a fire at the Vista Tower site at 363 E. Wacker Dr. The fire was contained to its basement office.

The building was unoccupied at the time, and the fire was extinguished by 7 a.m.