At the Junior World Championships in 2001 in Karpacz , Poland , Dementjew became world champion with the Russian 4x10 kilometer relay and finished seventh in the individual race over 30 kilometers. In the sprint, he finished 20th. At the Junior World Championships in 2002 in Schonach in the Black Forest , he was fifth in the sprint and took 18th place over 30 kilometers. He started for the first time in the World Cup in January 2003 in Kavgolovo and finished 13th in the 10 km freestyle. At the Nordic Junior World Ski Championships in 2003 in Sollefteå , he won bronze in the 30 km mass start race and gold over 10 km classic and with the Russian relay. He was tenth in the sprint. In February 2004 he reached his first podium in the World Cup with third place in the relay in La Clusaz and second place over 15 km classic in Umeå . In the 2004/05 season he came in 12 participations in World Cup individual races, six times in the top ten. At the season highlight of the Nordic World Ski Championships 2005 in Oberstdorf he won bronze with the relay, was 22nd in the 30-kilometer race with a change of style and reached 23rd place over 15 kilometers in free technique. At the end of the season he won his first World Cup victory in the 30 km pursuit race in Falun and finished the season in ninth place in the overall world cup and sixth place in the distance world cup. In the following season he again achieved a World Cup podium with second place in the 30 km mass start race in Lago di Tesero . At the 2006 Winter Olympics in Turin , he was Olympic champion in the pursuit race over 30 kilometers and won the silver medal in the 50 km race, while he was sixth with the Russian cross-country relay. At the beginning of the 2006/07 season he finished second in the relay in Gällivare and third in Cogne over 15 km classic. At the following World Cup in La Clusaz he was third in the 30 km mass start race and one day later he won his second World Cup victory with the relay. He finished the first Tour de Ski in 2006/07 in eighth place. At the Nordic World Ski Championships in Sapporo in 2007 he was able to win the silver medal in the relay together with Nikolai Pankratow , Vasily Rotschew and Alexander Legkow and finished 22nd in the individual race over 15 kilometers in the free technique. To kick off the World Cup but he was from the World Ski Federation FIS with a five-day protective barrier due to excessive hemoglobin been occupied. In the 2007/08 season he won third place with the relay in Beitostølen and ninth place overall in the 2007/08 Tour de Ski . In March 2008 he became the Russian champion in the 30 km pursuit race.
After Dementjew tested positive for the doping agent erythropoietin (EPO) in doping controls on January 2nd, 3rd and 19th, 2009 , he announced his retirement from competitive sport on August 26th, 2009. Before that he had participated in the World Championships in Liberec in March 2009 and finished 22nd in the 50-kilometer race. In November 2009 the FIS banned Dementjew for two years retrospectively from August 2009. After his ban had expired, however, he returned to competitions. He mainly takes part in the Eastern Europe Cup , which he finished eighth in 2012 and sixth in the overall ranking in 2015. At the Rollerski World Championships 2015 in Val di Fiemme , he won the silver medal in the 10 km mountain run and the gold medal in the 25 km mass start race. In the 2015/16 season he came second three times in the Eastern Europe Cup and won the overall standings at the end of the season. He also won the 15 km classic in the Alpine Cup in Hochfilzen . In March 2016 he won the Demino Ski Marathon . In 2018 he triumphed in the Gsieser Tal-Lauf classic over 42 km and achieved third place in the overall ranking of the Rollerski World Cup with a third and two first places . The following year he won the Ugra 50km freestyle ski marathon . In summer 2019 he came in second place in the overall ranking of the Rollerski World Cup with three victories .