In 2001-02, he played for one year with the Traktor Chelyabinsk hockey school. He split the season between their juniors team and their regular team, and during the season he scored 21 goals and made 8 assists between the two clubs.
The Capitals selected Semin with their first pick (13th overall) in the 2002 Entry draft; he decided to remain in Russia during the subsequent season. In 2002-03, he scored 10 goals and made 7 assists in 47 games with Lada Togliatti.
In the 2003-04 NHL season, Semin made his NHL debut. He scored 10 goals and assisted on 12 others during his stay in the NHL in 2003-04. However, he missed his team's flight to Pittsburgh when the Capitals closed out their season against the Pittsburgh Penguins. Semin then reported to Maine to play for the American Hockey League Portland Pirates, where he collected 15 points in four regular season and seven playoff games.
During the 2004-05 NHL lockout, Semin played 50 games for Lada Togliatti, scoring 19 goals, assisting on 11 goals, and compiling a Plus/minus rating of +15.
Semin began the 2005-06 season with Lada, but financial difficulties with the team resulted in a deal that brought him to Khimik (Chemist), a Russian Superleague team in Mytischi, just outside of Moscow. While with Lada, Semin played 16 games, accumulating 5 goals, 4 assists, and 52 penalty minutes.
Semin scored the first goal of the Capitals 2006-07 season against the New York Rangers on October 5, 2006. In his next game against Carolina he recorded a hat trick for the Capitals.
On March 18, 2007 Semin tallied the second natural hat trick of his career against the Tampa Bay Lightning, scoring three goals on three successive shots in a span of 7:04 in the second period.