World number one Carlos Alcaraz of Spain retired from his Paris Masters quarter-final against Holger Rune of Denmark on Friday with an abdominal injury, while defending champion Novak Djokovic swatted aside Italy's Lorenzo Musetti.

Alcaraz, the reigning U.S. Open champion, lost the first set and was trailing 3-1 in a second set tie-break when he decided to stop, having received treatment to his left side at the previous changeover.

"It's something wrong on the abdominal," said Alcaraz. "At the end of the set, it was wrong. It was getting wrong, and I preferred to retire and take care about it."

The Spanish teenager had been attempting to win a third Masters title of the year after victories in Miami and Madrid.

Alcaraz would have been confirmed as the year-end world number one before the Association of Tennis Professionals (ATP) Finals in Turin later this month had he lifted the trophy this weekend in Paris.

Instead, his participation at the November 13-20 event is now in doubt. Alcaraz had arrived in Paris struggling with a knee injury, an issue he downplayed earlier in the week.

"Right now, I have some test on how it's gonna be before Turin, but right now I'm focused to try to get better in the abdominal and trying to be at 100 per cent in Turin," said Alcaraz.

"I cannot stretch. I couldn't serve well. I couldn't hit the forehand well. When I turn the body, I feel it. I feel the abdomen in so many movements."

"It's in a zone that I have problems, I have problems before," he added. "Let's see if it's the same problem as I felt before or not."

Serbian Djokovic charged into the last four with a 6-0, 6-3 demolition of Italy's Lorenzo Musetti.

The 20-year-old Italian, who lifted the trophy in Naples last month, won just eight points as the Serbian motored through the first set in 24 minutes.

Djokovic, the 21-time Grand Slam champion gunning for a record-extending seventh Bercy title, dropped serve early in the second set but swiftly recovered with successive breaks.

"The tactic was to be very aggressive, to deny him time," said Djokovic. "It worked well."

Felix Auger-Aliassime stretched his winning run to 16 matches earlier in the day.

The 22-year-old Canadian continued his quest for a fourth title in as many weeks by defeating American Frances Tiafoe 6-1, 6-4 to power into the semifinals.

Auger-Aliassime also won in Antwerp and Basel last month. He is trying to achieve a feat no man has accomplished since Ivan Lendl won five tournaments in five weeks in 1981.

Having raced through the opening set against the 21st-ranked Tiafoe, Auger-Aliassime needed six match points to finally see off the American and reach his second Masters semifinal.

The eighth-seeded Auger-Aliassime has enjoyed a breakout season. His 56 wins are third most on the ATP Tour this year, while only Alcaraz has bettered his four singles titles.

"I have felt great not only this week but in the last few weeks," said Auger-Aliassime.

"Sixteen wins in a row is special. I have an opportunity to win again tomorrow. I'll try to seize it, but definitely it's been an amazing period."