World number one Carlos Alcaraz retired from his Paris Masters quarter-final against Holger Rune on Friday with an abdominal injury, while defending champion Novak Djokovic made quick work of Italy's Lorenzo Musetti.
Alcaraz, the reigning US 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 ATP Finals in Turin later this month had he lifted the trophy this weekend in the French capital.
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."
Rune will face in-form Felix Auger-Aliassime for a place in Sunday's final. The 19-year-old Dane lost to Auger-Aliassime in the final in Basel last weekend.
In the last match of the evening, Stefanos Tsitsipas eliminated American Tommy Paul, the American who dumped Rafael Nadal out, 6-2, 6-4.
"It was a very good match today, in terms of level and intensity, from start to finish," said the Greek world number five who will face titleholder Novak Djokovic in the semi-finals.
Tsitsipas dominated the first set. The second went with serve until 5-4 when the Greek, needing one game to take the match, broke to love.
Djokovic charged into the last four with a 6-0, 6-3 demolition of the 20-year-old Musetti.
Musetti, who lifted the trophy in Naples last month, won just eight points as Djokovic motored through the first set in 24 minutes.
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."
Earlier, Auger-Aliassime continued his quest for a fourth title in as many weeks by beating Frances Tiafoe 6-1, 6-4 to stretch his winning run to 16 matches.
The 22-year-old Canadian also won in Antwerp and Florence last month. No man has won four tournaments in four weeks since Ivan Lendl in 1981, on his way to winning five in a row.
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 semi-final.
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."