Armstrong began the day with a 2 minute, 46 second lead on Italian Ivan Basso, of the CSC team, with 1997 race winner Ullrich in fourth place, behind Michael Rasmussen, at nearly six minutes adrift of the six-time winner.
After Tuesday's stage -- which was the last real chance for any of his rivals to claw back time in the mountains -- Armstrong's lead remained the same as all the main challengers finished in the peloton although there are likely to be significant changes in the race's hierarchy after Saturday's 55.5km time trial in St. Etienne.
Ullrich, who is now riding to make sure he finishes on the podium, made some brief attempts to shake off Rasmussen and Armstrong on the 16.5km climb to the summit of the Col d'Aubisque. However, the T-Mobile team leader was quickly countered and soon brought under control.
At the front meanwhile Australia's Cadel Evans, who had attacked around eight kilometers from the summit of the unclassified Aubisque climb, was eventually caught on the tricky descent towards the day's final climb by Oscar Pereiro.
Evans, who rides for the Davitamon team and is fighting for a top ten finish on his race debut, had broken away from his group of earlier breakaway companions with speed which left all of them trailing. The 27-year-old former mountain bike champion came over the summit with a 48-second lead on Pereiro, with Italian Eddy Mazzoleni of Lampre not far behind.
Armstrong's group came over the Aubisque summit over four minutes behind Evans, who saw his chances of a stage victory compromised when he was caught by Pereiro and Mazzoleni after just a few kilometers of the descent.
Pereiro has been one of the main race protagonists in the mountains, and was pipped by Armstrong's Discovery Channel teammate George Hincapie in what was a prestigious stage victory on Sunday when the American won on the summit finish of Pla d'Adet after racing over five Cols. The Spaniard appeared unwilling for the same to happen again, especially by a race debutant, and he soon overtook Evans.
However the Phonak rider's determination led to brief disaster. He punctured shortly after racing past Evans, allowing Mazzoleni and the Australian to coast past him as he stood frustrated with his bike waiting for a spare back wheel.
Evans and Mazzoleni were soon rejoined by Pereiro however, and Iles Baleares rider Xavier Zandio who like Pereiro and Mazzoleni had tried to limit Evans' lead on the climb.
The four riders appeared to be collaborating, for the meantime, and were managing to hold off a five-man group who were over two minutes behind them while Armstrong's group appeared to be happy keeping an eye on each other at over four minutes behind.
Pereiro was, though, to prevail and grab the stage victory he merited as he outsprinted compatriot Xabier Zandio while Mazzoleni took third and Evans a superb fourth.Javier Soriano/AFP/Getty Images Oscar Pereiro battles back after a flat tire to finish just ahead of Spanish Xabier Zandio
Top 10 results, Stage 16: Mourenx - Pau (180.5 km)
1. Oscar Pereiro (ESP/PHO) 4:38:40
2. Xavier Zandio (ESP/BAL) at 0:00
3. Eddy Mazzoleni (ITA/LAM) 0:00
4. Cadel Evans (AUS/DAV) 0:00
5. Philippe Gilbert (BEL/FDJ) 2:25
6. Anthony Geslin (FRA/BOU) 2:25
7. Jorg Ludewig (GER/DVE) 2:25
8. Juan Antonio Flecha (ESP/FAS) 2:25
9. Ludovic Turpin (FRA/A2R) 2:25
10. Cedric Vasseur (FRA/COF) 2:25
Stage 16: Complete results
Top 10, Overall standings
1. Lance Armstrong (USA/DIS) 66:52:03
2. Ivan Basso (ITA/CSC) at 2:46
3. Michael Rasmussen (DEN/RAB) 3:09
4. Jan Ullrich (GER/MOB) 5:58
5. Francisco Mancebo (ESP/BAL) 6:31
6. Levi Leipheimer (USA/GRL) 7:35
7. Cadel Evans (AUS/DAV) 9:29
8. Floyd Landis (USA/PHO) 9:33
9. Alexander Vinokourov (KAZ/MOB) 9:38
10. Christophe Moreau (FRA/C.A) 11:47
Stage 16: Complete overall standings
Stage 16: Category standings