Raonic and fellow Canadian Vasek Pospisil both advanced to the semifinals at the men's event in Montreal, marking the first time in the Open Era that two Canadians will meet in the semis.
Sampras, in Toronto for a Legends Cup exhibition event with fellow former world No. 1s Jim Courier and John McEnroe, says the No. 13-ranked Raonic is due for a breakthrough but it won't come easily.
"He's up against (Novak) Djokovic and (Andy) Murray and Roger (Federer) and four great players, and it takes time," said Sampras. "I think he's a threat already. I think he's got a huge game, monster serve, willing to come in and do some things and get to the net."
Sampras is second all-time in Slams won (14) and weeks spent as the world’s No. 1-ranked player (286). He won two Australian Opens, seven Wimbledon titles, and five U.S. Opens.
He exited pro tennis by winning his very last match in the 2002 U.S. Open final against Andre Agassi.
Sampras reflected back on his own career and the time it took him to find the kind of success that has so far eluded Raonic in the majors.
"I didn't figure this game out until I was probably 22 or 23, so when everything was settled in I was physically and mentally great. I knew where I stood in the game," he said. "It just takes time. I know Milos does the right things; he works hard; he wants to do well.
"He just has to be patient. It isn't going to happen overnight."
The 41-year-old Sampras is often asked about the state of the American game. When the new ATP ranking are released, there will be no American men in the top-20 for the first time.
"People ask me all the time what's wrong with American tennis. I don't know why," he said. "It's looking a little slim. We're in decent shape, but obviously we want our guys to be ranked No. 1 like we were in the 90's.
"The world has gotten a little better. Maybe the U.S has gotten a little bit complacent. I really don't know. I think what happened in the 90's with Andre, me and Jim was rare."