Menchov swept through in the last 500m to win stage 5
Russian Denis Menchov sprinted clear of Danilo Di Luca, the previous day's winner, to claim the fifth stage of the Giro d'Italia in the Dolomites.
The Rabobank rider won by two seconds but Di Luca took the overall lead from Thomas Lovkvist, who finished three seconds behind the Italian in third.
Britain's Bradley Wiggins finished 22nd while Mark Cavendish suffered technical problems and ended 192nd.
Lance Armstrong crawled home almost three minutes off the pace.
A breakaway was established 5km from the start in San Martino with Barloworld's Francesco Bellotti and Astana's Eros Capecchi at the forefront.
Francesco Gavazzi and France's Thomas Voeckler helped push the pace and by 76km an eight-rider group had a four minute 43 second advantage over the main bunch.
But as the leaders reached the uphill stretch 28km from home, they saw their lead gradually chewed up by a hungry chasing pack and with 10km to go they were swallowed up by the bunch.
Giampaolo Cheula was then casually hauled back in when he attempted an attack.
Garmin-Slipstream's Wiggins showed he is developing a head for the mountain heights as he clung onto the back of the front group in a gutsy performance, but Armstrong faded badly as the riders became stretched in the closing stages and lost contact with 6km to go.
Only the escort of Astana team-mates Janez Brajkovic, Daniel Navarro Garcia and Jose Luis Rubiera kept the Texan from slipping further than 22nd in the overall standings.
Ivan Basso hit the front with 3km to go, before passing the responsibility to Tour de France champion Carlos Sastre.
But Menchov had the most left in reserve and broke Di Luca's resolve before coasting over the line.
Thursday promises to be another gruelling day as the riders tackle the 248km (154 mile) route from Bressanone to Brixen Mayrhofen.
Stage Five result:
1. Denis Menchov (Russia / Rabobank ) 3hrs 15mins 24secs
2. Danilo Di Luca (Italy / Team LPR ) +2secs
3. Thomas Lovkvist (Sweden / Columbia ) +5secs
4. Ivan Basso (Italy / Liquigas ) same time
5. Levi Leipheimer (U.S. / Astana ) +9secs
21. Charles Wegelius (Great Britain /Silence) +1min 47secs
22. Bradley Wiggins (Great Britain / Garmin) same time
35. Lance Armstrong (U.S. / Astana) +2min 58secs
98. David Millar (Great Britain / Garmin) +13mins 48secs
192. Mark Cavendish (Great Britain / Columbia) +17mins 48secs
1. Danilo Di Luca (Italy / Team LPR ) 16hrs20 mins 44secs
2. Thomas Lovkvist (Sweden / Columbia ) +5secs
3. Michael Rogers (Australia / Columbia ) +36secs
4. Levi Leipheimer (U.S. / Astana ) +43secs
5. Denis Menchov (Russia / Rabobank ) +50secs