Given how close things are at the top of the general classification, more attacks were anticipated during Thursday's stage, a 196.5km hike through the hills of Monts du Forez.
But none of the big five tried their luck and most of the riders rolled over the line together.
"I had quite a relaxing day on the bike," Sastre said.
"I don't want to think about the time trial. I just want to enjoy spending each day in the yellow jersey."
Lampre team-leader Damiano Cunego had a more difficult time and had to fight to stay within the time limit
The Italian was left dazed and grazed by an early crash and never regained touch with the peloton, eventually finishing around 12 minutes after the main group thanks to support from three of his team-mates.
Apart from Cunego's battle against the clock, the only real excitement came in a tense finish between Burghardt and Barredo - who had been out front together for three hours.
Barredo attempted to outwit his stronger rival and at times the pair almost slowed to a complete stop before Burghardt exploded clear a few metres before the line.
Burghardt, like British sprinter Mark Cavendish, is a member of the Columbia team and his success gave them their fifth stage win of what has been a memorable Tour.
"After all the work he did to help the team during the race Marcus got his chance today, and his win is just rewards for his contribution," said Columbia team manager Bob Stapleton.
"I was just hoping that he had enough energy left in the finale after everything he's done in the Tour."
Stage 18 results, Bourg d'Oisans to Saint Etienne (196.5km):
1. Markus Burghardt (Germany/Columbia) 4hrs 30 min 21sec 2. Carlos Barredo (Spain/Quick-Step) same time 3. Romain Feillu (France/Agritubel) +3min 33sec 4. Christophe Le Mevel (France/Credit Agricole ) same time 5. Mikel Astarloza (Spain/Euskaltel) +3min 35sec 6. Samuel Dumoulin (France/Cofidis) +6min 39sec 7. Cyril Dessel (France/AG2R ) same time 8. Roman Kreuziger (Czech Republic/Liquigas) same time 9. Leif Hoste (Belgium/Silence-Lotto) same time 10. Andy Schleck (Luxembourg/CSC) same time
48. David Millar (GB/Garmin) +6min 50sec
Overall standings (after 18 stages):
1. Carlos Sastre (Spa/CSC) 79hrs 16min 14sec 2. Frank Schleck (Lux/CSC) +1min 24sec 3. Bernhard Kohl (Aut/GST) +1min 33sec 4. Cadel Evans (Aus/SIL) +1 min 34 sec 5. Denis Menchov (Rus/RAB) +2min 39sec 6. Christian Vandevelde (US/GAR) +4min 41sec 7. Alejandro Valverde (Spa/GCE) + 5min 35sec 8. Samuel Sanchez (Spa/EUS) + 5mins 52sec 9. Tadej Valjavec (Slovenia/AG2R) +8min 10sec 10. Vladimir Efimkin (Russia / AG2R ) +8min 24sec
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