Pospisil had to deal with the heat and some niggling injuries on the way to a 6-7 (3), 7-6 (4), 6-3, 6-4 win over Italy's Paolo Lorenzi to reach the third round in Melbourne for a second consecutive year.
The eighth-seeded Raonic, meanwhile, looked strong in a 6-4, 7-6 (3), 6-3 victory over American Donald Young. Raonic, from Thornhill, Ont., fired 17 aces and had 44 winners.
"I served well and got better and better as the match went on," said Raonic. "I was much more aggressive than I was in the first round. That was a step forward."
"I've put in a lot of good work, I believe. I'm just very by the book in the things I want to do. I'm just trying to follow that through."
In five appearances, Raonic has never failed to reach at least the third round in Melbourne. He next faces German Benjamin Becker, who came from two sets down to defeat Australia's Lleyton Hewitt 2-6, 1-6, 6-3, 6-4, 6-2.
Pospisil, from Vancouver, duplicated his best Grand Slam singles showing with his win in just over 3 1/2 hours. He said he had to fight the elements to stay in the contest.
"I didn't feel that great. It was a tough match," he said. "It was surprisingly more humid than I thought it would be. My opponent was tough and conditions were tough. But I found a way to win somehow.
"I was not feeling as good as I should have been feeling, and was not hitting as big as I wanted."
Pospisil had 22 aces despite getting treatment on court at various times for leg and back issues. The world No. 60 next faces Spain's Guillermo Garcia-Lopez, who defeated Alejandro Gonzalez of Colombia 6-1, 6-3, 6-3.
The Canadian finished almost level on stats, with 66 winners and 64 unforced errors. He broke Lorenzi on four of 10 occasions.
Pospisil said that he caught a break when the sun dipped behind the clouds and some breeze kicked up after he dropped the 57-minute first set.
"The cloud cover and wind helped me a lot," he said. "I was able to catch my breath. That changed things a bit for me and helped me win the second set. In the end it was more of a physical battle than a tennis battle."
Pospisil played down his injury treatments.
"I had some hip pain but only for a few games," he said. "I had a small recurring back issue — a muscle which has been tightening up on me for a few weeks occasionally. But not anything big, nothing that would be match-threatening."
Pospisil won a five-setter on Tuesday and then played doubles a day later in a full program at the first major of the season. He's looking forward to his first career meeting against Garcia-Lopez.
"This is a good opportunity for both of us," Pospisil said. "He's a great player with a lot of experience. It's better to play him than a lot of guys I could be meeting in the third round of a Grand Slam."
Toronto veteran Daniel Nestor and new partner Rohan Bopanna of India opened the first round with a 7-6 (2), 7-5 victory over Marcos Baghdatis and Australian Marinko Matosevic. Nestor and Bopanna were victorious in Sydney last week.