The Wimbledon champion stretched his winning streak to 14 matches, but was tested by his opponent, who is ranked No 116 in the world.
"He's always played me very tough." said world the No 2. "He's much better than his ranking. I've played him three times now and he's taken me the distance. He's got nothing to lose and has the ability."
Sampras will meet Wayne Ferreira today. The eighth-seeded South African was two points from defeat but managed to beat Justin Gimelstob, of the United States, 5-7 7-5 6-2.
"Wayne is an unbelievable talent and when he gets his game going he's very tough to beat," said Sampras, who despite holding a 5-4 career record over Ferreira has lost the last four meetings. "He's proven he's a top 10 player and he's tough to beat. But I'll be ready for him."
Chang, the former world No 2, is now down to No 70 in the world after injuries to his knee, wrist, and back last year.
"I feel like it's a good start," Chang said after reaching his second quarter-final of the year. "It's been tough for a while, so it's going to take a little bit of time to get confidence back."
Chang wasted little time despatching Raoux. "It was a good match for me," he said. "Guillaume is a dangerous player from all parts of the court. I was pleased with the way things went."
Raoux had been suffering from diarrhoea, but he did not use it as an excuse for the early dismissal.
"Michael played too good today," he said. "His ball has no pace, it's like hitting a ball of cotton. It's really tough to hit and makes the opponent play very bad."
Australian qualifier James Sekulov eliminated the fifth-seeded, two-time finalist Thomas Enqvist of Sweden 6-7 6-4 6-3 to reach his first Tour quarter-final.
"It feels good to get the win," said Sekulov, who at No 236 in the ATP Tour rankings was playing in only the seventh main draw of his six-year career. "I hung in there pretty well - obviously he wasn't playing his best. I just kept fighting.
"Being in the quarters is pretty hard to believe because of the way I was playing last week," added Sekulov, who plays most of his tennis in Challenger events.
Enqvist, who was playing in his first tournament since Wimbledon last month said, "It's been a long time since I've been so frustrated out on the court. Nothing was working. I can't remember the last time I played so bad."