In 2001, Semenov was awarded the Max Kaminsky Trophy as the best defenceman in the Ontario Hockey League (OHL). After playing three full seasons for the Oilers and their top farm club, the Hamilton Bulldogs, he was traded to the Florida Panthers for a conditional draft pick in the 2006 NHL Entry Draft on November 19, 2005. Upset at his assignment to the Rochester Americans at the start of the 2006–07 season, Semenov left Rochester after four games to play for Salavat Yulaev Ufa in Russia. On July 27, 2007, Semenov signed a one-year contract with the San Jose Sharks.
Semenov was invited to training camp with the New York Rangers for the 2009–10 season and was offered a one-year contract before the season to serve as the team's seventh defenseman. Semenov turned down the contract based on the amount of money offered, countering Rangers General Manager Glen Sather's claim that Semenov's wife talked him out of it after he had already agreed to terms. Subsequently, less than a week later, Semenov signed a two-year contract worth $1.5 million with Dynamo Moscow. The following season, Semenov was again invited to the Rangers training camp, and accepted.