Pospisil was left fuming Tuesday after a tough 4-6, 3-6, 7-6 (9), 6-2, 7-6 (10) loss to Brazilian qualifier Rogerio Dutra.
With the fifth-set tiebreaker tied 10-10, the umpire overruled a linesperson's call on Dutra's shot that was initially ruled long. That gave Dutra match point, which he then converted to complete the win over the native of Vernon, B.C.
"It was a clear mistake, a huge mistake," Pospisil said. "That ball was 2-3 inches long. There was no doubt — the next thing he called it in. Something has to be done about mistakes like that.
"To overrule on the baseline is absolutely terrible. The linesperson called it out, it was not even close. I was in a state of shock and anger. I've never experienced a call like that, especially at this stage of a grand slam match.
"It doesn't excuse that I didn't convert leading to that point, but I would have had another match point and maybe I would have gotten it."
The match was played on Court 14 where replay challenges are not available. It was completed after being delayed by rain Monday night with Pospisil up two sets to one, but trailing the fourth set 4-0.
"In the third set I had stopped serving well, I had nothing in my legs and I could feel my body going downhill," he said. "I had cramps but I felt better today."
The 10th-seeded Raonic from Thornhill, Ont., had a lot less drama in his opener as he defeated Italian qualifier Thomas Fabbiano 6-3, 7-6 (6), 6-3.
"I know he's very capable, I remember him from juniors," Raonic said of Fabbiano. "He got through quallies, so he must be playing well in that situation. I just sort of dealt with it as best I could, and I'm happy with how the result turned out."
Raonic will face Pablo Andujar in the second round after the Spaniard beat Thiemo de Bakker of the Netherlands 6-4, 6-4, 6-4.
In the women's draw, Aleksandra Wozniak of Blainville, Que., reached the second round with a win over Serbia's Vesna Dolonc 7-5, 7-6 (7-5).
Wozniak hadn't played in a Grand Slam since 2012 due to a shoulder injury and had only played two matches in the past year. She was pleased to be back on the court.
"I'm very happy to be back, it was ten solid months of rehab at home. Six months of no tennis was the longest of my career," said Wozniak. "I'm finding form a lot quicker than I thought I might and I'm really pleased with this win."
Raonic put on a serving display against the 179th-ranked Fabbiano, who was making his Grand Slam debut. The Italian gave Raonic only the slightest hint of trouble when he got a break back in the second set for 4-all before losing the set in a tiebreaker.
Raonic took a 4-2 lead in the third set and sealed the victory with a series of untouchable serves. He finished with 52 winners, including 28 aces.
"Serve helped for sure, got me get through a lot of difficult situations," Raonic said. "I was struggling definitely from the baseline, but when I sort of needed it I was able to pick it up on both sides.
"In general, it was a good day."
Raonic sees some parallels between his next opponent Andujar and himself.
"He started doing well in 2011, just like me. He's very consistent. He can be aggressive, especially with his forehand, and I've got to play more aggressive than I did today," he said.
"I've got to clean up a bit off the baseline and hopefully serve like I did today."