'Very sad' Sinner withdraws from Madrid Open with hip injury
"Very sad to have to withdraw from my next match here in Madrid," Sinner wrote on X.
"My hip has been bothering me this week and has slowly been getting more painful. Taking the advice from the doctors we decided it's best to not play further and make it worse."
Sinner's withdrawal came just minutes after world number three and two-time defending champion Carlos Alcaraz was knocked out in the quarter-finals by Andrey Rublev.
Italian Sinner, 22, who was making his third appearance in Madrid, holds a 28-2 record on the season.
He lifted his maiden Grand Slam title at the Australian Open in January before claiming his second Masters 1000 crown in Miami.
Sinner is due to play on home ground at the Italian Open in Rome next week before heading for an assault on the French Open, the season's second major, which starts in Paris on May 26.
World number 35 Auger-Aliassime will face either third seed Daniil Medvedev or 30th seed Jiri Lehecka for a place in the final.
Lehecka knocked out five-time champion Rafael Nadal in the Madrid fourth round on Tuesday.
Sinner had appeared to struggle with a hip problem during his third-round win against Pavel Kotov, and the Italian was not fully recovered against Karen Khachanov when he defeated the Russian in three sets on Tuesday.