Ekaterina Gamova, a 2.02 metre-tall opposite spiker, added 31 points in the 26-28, 25-19, 22-25, 25-16 15-11 victory for ninth-ranked Russia. Simona Gioli had 21 points for the No. 4 Italians.
Both teams were undefeated going into the match, and both had dropped just two sets. The teams had also already earned spots in the quarterfinals, so the match decided pool finish.
Evgeniya Startseva's misplay gave Italy set point in the first but Russia roared back to easily take the second. Another Russian error gave the Italians the third.
Russia went up 18-8 to cruise through the fourth and force the deciding set. Down 13-9, Italy scored three straight points, but Gamova's spike and Gonhcharova's block won it.
Sunday was the final day of preliminary-round play at London's Earls Court. Russia and Italy were joined by Japan and the Dominican Republic in advancing out of the pool. Britain and Algeria were eliminated.
The United States had already clinched the top spot in the other group, while China and South Korea earned berths. The final team to advance was to be determined in Sunday night's final matches between the United States and Turkey, and Brazil and Serbia.
The top seeds will play the fourth-ranked pool teams, while lots are drawn for the other matches, which start Tuesday.
Japan defeated Britain 3-0 (25-19, 25-14, 25-12), eliminating the home team from the Olympic competition.
The 69th-ranked British women, who earned a spot in the Olympics as the hosts, surprised the field with a five-set victory over Algeria earlier in the preliminary round. Britain was not expected to win many sets, let alone a match.
The British, who had to raise money themselves to train and practiced in a small fire station gym, were given a sustained standing ovation from the crowd at Earls Court after Sunday's loss. One held up a hand-lettered poster that read "Thank You!"
"It has been unforgettable," British middle blocker Ciara Michel said. "We've been playing against the best teams in the world. It has been a phenomenal experience."
China defeated South Korea 3-2 with both teams earning a spot in the quarterfinals because of the five-set result.
A five-set match was the only way mathematically for both teams to advance to the quarterfinals. Had one team won all three points, the other team would have had to depend on results later in the day.
China won it 28-26, 22-25, 25-19, 22-25, 15-10, taking match point on Xu Yunli's kill.
Both coaches emphatically denied that they had purposely pushed the match to five sets to ensure a quarterfinal berth for both teams.
The Dominican Republic defeated Algeria 25-15, 25-16, 25-13 to advance. Algeria, winless in London, was eliminated.