"It was one of those days you wish you had every day," the top-ranked Sampras said after breezing through in 51 minutes against an ailing Agassi, his fourth straight victory over his rival. "My tennis was the best I've played in a long time."
Agassi said he fell ill two days before coming here with a combination of an upset stomach and influenza. "I am still feeling weak, I wasn't fit to play. It wasn't enjoyable to go and play out there," he said.
Agassi later decided to withdraw from the tournament, bringing in Sweden's Thomas Enqvist, who was the first alternate in the event for the top eight players in the world.
Sampras conceded that Agassi may have had an off day, but said his own tennis had something to do with Agassi's poor performance. "I was in a zone, I couldn't play any better. It was one of those days when everything clicks," he said.
Sampras dominated the match and the crowd of 15,000 whistled and booed Agassi. "I thought they were pretty hard on Andre. I felt he was still competing out there," Sampras said.
Sampras's victory was similar to his win over Agassi in Stuttgart last month. This time, though, Sampras was even more devastating, winning all of his 19 first-serve points. "This maybe outdoes it. I dropped my serve a couple of times in Stuttgart, I didn't today."
Agassi needed six deuces to hold serve in the fourth game. Over the next nine games, he won six points. Sampras was all over him, serving aces, hitting fearsome returns and firing winners from all over the court. "I've had spurts of tennis like this, but not for an entire match," he said.
Agassi, a former world No 1 who has now dropped to No 7, has not beaten Sampras since July 1995 in Montreal.
Earlier, Thomas Muster stayed alive in the tournament by beating Michael Chang 6-4, 6-3, leaving the American world No 2 and last year's runner- up without a victory in two matches.
Chang has little chance of advancing from his round-robin group, while Muster is now 1-1, following his opening defeat to Ivanisevic.
Muster came into the tournament with only two indoor wins this year, but he dominated Chang from the beginning, breaking the American's serve for a 2-1 lead.
Muster had won only one of his seven matches in previous appearances at the ATP season-ending event. But the former French Open champion was on top of his game.
"It was nice to be able to play tennis, to get a chance to run and fight for the balls. Yesterday, it was a completely different situation," said Muster, who was beaten easily by Ivanisevic.
"Michael wasn't as aggressive today as usual and he made some unforced errors. I served well in important moments, I put him under pressure, especially at the end," Muster said.
Ivanisevic later became the first player to reach the last four after a 6-4, 6-7, 7-6 victory over the Wimbledon champion, Richard Krajicek, his second win in the White Group.
Results, Digest, page 31