The Fitness Center at Cosmos Selection Saint-Petersburg Nevsky Royal Hotel, a Member of Radisson Individuals
Radisson Royal Hotel, St.Petersburg's fitness center is mentioned by those who travel for a living. Out of 4 professionals that we monitored, 2 referenced the fitness center, including Fodor's and Lonely Planet.
Cosmos Selection Saint-Petersburg Nevsky Royal Hotel, a Member of Radisson Individuals 
Fodor's
Lonely Planet
Fodor's "A prime spot on the corner of Nevsky prospekt and Vladimirsky prospekt is the choicest real estate in town, close to everything, and within the historic building itself a range of amenities is on offer, including a restaurant, bar, and a nice gym"
Lonely Planet "The gym is small but does offer a Jacuzzi and Finnish sauna, and it's possible to book a massage"
