Carlos Alcaraz says he "preferred" to retire from his Paris Masters quarter-final than risk further damage to an abdominal injury before the ATP Finals.

It leaves the Spaniard with a fitness concern just days before next week's season-ending ATP Finals in Turin.

"I cannot stretch. I couldn't serve well, I couldn't hit the forehand well," said US Open champion Alcaraz.

"I preferred to retire, see the extent of it and take care.

"I'm focused on trying to get better in the abdominal and trying to be at 100% [for the ATP Finals] in Turin."

Alcaraz is set to make his debut at the eight-man ATP Finals, which is indicative of his rise over the past 12 months.

He will also be hoping to be fully fit for the Davis Cup Finals, which take place in the Spanish city of Malaga at the end of the month.

Rune, 19, had outplayed Alcaraz before the top seed had to retire and will play Canada's Felix Auger-Aliassime in the semi-finals.

Auger-Aliassime, who has won titles in Florence, Antwerp and Basel recently, extended his winning run to 16 matches after racing to a 6-1 6-4 win over American Frances Tiafoe.

"It's unlucky for [Alcaraz]," said Rune, who is also in form after winning 17 of his past 19 matches.

"I think we both played very well and I was super focused.

"It was an amazing crowd, amazing tennis, so I'm super pleased with how I handled everything today."

Serbia's 21-time major champion Novak Djokovic is aiming to book his spot in the Paris last four when he plays Italian youngster Lorenzo Musetti later on Friday.

American Tommy Paul, who beat 22-time major champion Rafael Nadal in the first round, faces Greek fifth seed Stefanos Tsitsipas in the other quarter-final.