The 25-year-old High Road rider began the day in eighth place, 6 seconds behind Trent Lowe, of Australia. Sivtsov passed Lowe in the steep final half-mile to win the 88.4-mile Blairsville-to-Brasstown Bald Mountain sixth stage in 3 hours, 47 minutes, 16 seconds. Lowe (Slipstream-Chipotle) was second, 10 seconds behind.
Race favorite Levi Leipheimer (Astana) was third, 16 seconds behind Sivtsov. With one stage left in the 529-mile event, Sivtsov had a 4-second lead over Lowe.
Leipheimer, the American who rode to a dominating win in the mountain stage last year, was third overall, trailing Siutsou by 16 seconds.
"I waited and waited until the final half-mile; my teammates and team director told me it was the hardest part and just waited and waited," said Sivtsov, a former amateur world road winner.
Lowe, who assumed the race lead after the fifth stage, and Leipheimer, the reigning Tour of California champion who placed third in the 2007 Tour de France, were expected to vie for the stage that ended at 4,783 feet, the highest elevation in Georgia.
After finishing second in the stage, Lowe thought he had retained the overall race lead. Nearly an hour later, race officials announced the results.
"He (Sivtsov) rode a strong race, but I was confident in my abilities and I rode at my limit," Lowe said. "At the end, we were all at our limits."
Lowe and Leipheimer exchanged aggressive riding tactics over the final 2 miles. But both riders underestimated Sivtsov, who finished 32nd overall in last year's Tour de France.
"I think I made some mistakes, but it was a really tough call," said Leipheimer, from Butte, Mont. "I thought if we made it really hard at the bottom (of the final climb), my teammates could crack everyone for me. I felt strong, but Trent (Lowe) and especially Sivtsov were also very strong."
The severity of the sixth stage, the only mountain stage of the race, depleted the field. The final finishers were 24 minutes behind the winner.
Despite his slim leading margin, it's unlikely Sivtsov will lose the race lead in the final stage. The stage includes mid-stage and stage finish bonus seconds for top placings. But the time likely will be awarded to sprinters, not riders contending for the overall title.
Brasstown Bald Mountain, Ga.
88.4-mile Blairsville to Brasstown Bald road race
1. Kanstantin Sivtsov, 3 hours, 47 minutes, 16 seconds.
2. Trent Lowe, 3:47:26.
3. Levi Leipheimer, 3:47:32.
4. Rory Sutherland, 3:47:49.
5. Oscar Sevilla, 3:47:54.
6. Moises Aldape Chavez, 3:48:10.
7. Iigo Cuesta, 3:48:20.
8. Antonio Colom, same time.
9. Christian Vande Velde, 3:48:54.
10. Glen Chadwick, 3:49:36.
Overall Standings (After six stages)
1. Kanstantin Sivtsov 20 hours, 20 minutes, 51 seconds.
2. Trent Lowe, 4 seconds behind.
3. Levi Leipheimer 14 seconds behind.
4. Antonio Colom 1 minute, 2 seconds behind.
5. Ligo Cuesta 1:11 behind.
6. Oscar Sevilla, 1:25 behind.
7. Christian Vande Velde, 1:32 behind.
8. Moises Aldape, 1:37 behind.
9. Rory Sutherland,, 2:08 behind.
10. Bobby Julich,, 2:31 behind.