The fifth stage win is the biggest of Menchov's career

The Russian withstood a spirited challenge from Spaniard Aitor Gonzalez of the Fassa Bortolo team to claim victory on the 186.5km stage.

Another Spaniard, Alejandro Valverde, riding for the Comunidad Valenciana team, secured third place.

US Postal rider Manuel Beltran, yet another Spaniard, took over as overall leader in the race.

The T-Mobile team endured a tortuous day in the saddle after a bout of food poisoning swept through the camp.

Team leader Alexandre Vinokourov finished more than 17 minutes down, while three of his team-mates, including Santiago Botero, pulled out of the race.

Menchov, 26, moved up to third in the overall standings and said: "Our team time trial in Zaragoza was a good start and this is a good second step too.

"I wanted to try and get some distance between myself and Valverde before he attacked but then saw I had enough of a gap to win."

Beltran, ninth in the stage, is level on time overall with his team mate and the race's first leader, Floyd Landis, but takes over the lead because of better stage placings.

"Tomorrow it could be Floyd giving the leader's press conference not me but in any case there are many other contenders in better form than us," he said.

"You have to remember that both of us rode the Tour this summer, and that could mean we tire earlier."

Fifth stage result:
1. Denis Menchov (Russia) Illes Balears Banesto, 5 hours, 7 mins, 44 seconds
2. Aitor Gonzalez (Spain) Fassa Bartolo, 3 seconds behind
3. Alejandro Valverde (Spain) Kelme, same time
4. Leonardo Piepoli (Italy) Saunier Duval, same time
5. Carlos Sastre (Spain) Team CSC, 5 seconds behind
6. Cadel Evans (Australia) T-Mobile, 9 seconds behind
7. Francisco Mancebo (Spain) Illes Balears Banesto, same time
8. Isidro Nozal (Spain) Liberty Seguros, same time
9. Manuel Beltran (Spain) US Postal, same time
10. Angel Gomez (Spain) Costa de Almeria, same time

Overall standings:
1. Manuel Beltran (Spain) US Postal, 18:47:16
2. Floyd Landis (US) US Postal, same time
3. Denis Menchov (Russia) Illes Balears Banesto, 4 seconds behind
4. Alejandro Valverde (Spain) Kelme, 9 seconds behind
5. Cadel Evans (Australia) T-Mobile, 16 seconds behind
6. Benoit Joachim (Luxembourg) US Postal, 28 seconds behind
7. Francisco Mancebo (Spain) Illes Balears Banesto, 39 seconds behind
8. Carlos Garcia Quesada (Spain) Kelme, 47 seconds behind
9. Tyler Hamilton (US) Phonak, 50 seconds behind
10. Victor Hugo Pena (Colombia) US Postal, same time