Dementieva played and won her first international tournament, Les Petit As, in France at the age of 13. In 1997, she entered the WTA top 500. She turned professional in 1998 and entered the top 100 in 1999.

In 2008, at the Medibank International tournament in Sydney, Dementieva lost in the first round to Sofia Arvidsson 6–1, 7–5. Dementieva then reached the fourth round of the Australian Open in Melbourne before losing to eventual champion Maria Sharapova6–2, 6–0. Dementieva then travelled to Paris for the Tier II Open Gaz de France indoor tournament. She reached the semifinals before succumbing to seventh-seeded Agnes Szavay from Hungary 6–3, 1–6, 7–5. Dementieva then won the Tier II, US$1.5 million Barclays Dubai Tennis Championships, defeating second-seeded compatriot Svetlana Kuznetsova in the final. This was Dementieva's fourth career Tier II tournament title but her first since 2006. Along the way, she defeated Patty Schnyder, Alona Bondarenko, World No. 2 Ana Ivanovic, and Francesca Schiavone (who defeated World No. 1 and defending champion Justine Henin in the quarterfinals). At the Tier I Sony Ericsson Open in Key Biscayne, Florida, Dementieva lost in the quarterfinals to Jelena Jankovic. This performance caused her ranking to rise to World No. 8. At the Tier I Family Circle Cup in Charleston, South Carolina, Dementieva lost in the semifinals to compatriot Vera Zvonareva. Her ranking then dropped to World No. 9. Dementieva was the seventh-seeded player at the Tier I Qatar Telecom German Open in Berlin but reached the final, where she lost to thirteenth-seeded Dinara Safina. Dementieva defeated fourth-seeded Jankovic in the quarterfinals and second-seeded Ivanovic in the semifinals. Dementieva's win over Ivanovic was her fourth in four career matches with Ivanovic.

Her next tournament was the Istanbul Cup in Istanbul, Turkey where Dementieva was the top seed and defending champion. She reached the final but was unsuccessful in her attempt to win a singles title at the same tournament in consecutive years, which would have been a career first. Agnieszka Radwanska of Poland defeated Dementieva in the final.

At the French Open, Dementieva defeated eleventh-seeded Zvonareva in the fourth round 6–4, 1–6, 6–2 but lost her quarterfinal match against Safina 4–6, 7–6(5), 6–0 after Dementieva had a match point while leading 5–2 in the second set. Dementieva became the World No. 5 player on June 9, 2008.

At the grass court Ordina Open in 's-Hertogenbosch, Netherlands, Dementieva was the top-seeded player but lost in the semifinals to Safina 6–3, 6–2. Dementieva is seeded fifth at the ongoing Wimbledon Championships. She defeated Maria Elena Camerin in the first round 6–3, 6–7, 6–3.