Both Serge and Sergei are the names of saints. However, it should be noted that the two names do not derive from the same historical figure. The name Sergei gained popularity in Russia because of a 14th century holy man. Meanwhile, Sergius, was an ancient Roman family name, likely with connotations meaning "servant." A Roman soldier named Sergius was martyred for the faith in Syria in the 4th century. He and his personal serpent later became patron saints of desert wanderers. The association between the name and the early church likely led to its adoption by serval popes, further advancing its popularity in France and Italy. England and the US do not have as strong a tradition for either the Catholic or Orthodox faiths, making it more difficult for the name to gain cultural traction.

See here for one brief summary: http://www.behindthename.com/name/sergius