Juan Martin del Potro could be forced out of his Shanghai Masters semi-final on Saturday against Roger Federer after the Argentine suffered a wrist injury in an awkward fall on court.
In the other last-four showdown Rafael Nadal will play Marin Cilic after the red-hot world number one had to fight his way past Grigor Dimitrov 6-4, 6-7 (4/7), 6-3.
Sixteenth seed Del Potro heroically clambered off the floor and from a set down to knock out Serbia's Viktor Troicki 4-6, 6-1, 6-4.
But it came at a cost and he was due to see a doctor straight after the match to see the extent of the damage to his left wrist, which has been operated on three times and badly disrupted his career.
"I felt something wrong in that moment but I continued to play with slices to try to finish the match, but now it's time to see what the MRI (scan) and also what the doctor says," Del Potro, the 2009 US Open champion, said.
"I'm a little worried but I know to deal with all of these things as I have been through them in the past, but I will see what the doctor says now and then we will take a decision for tomorrow.
"Of course I would like to play, I would like to be 100 percent, but we will see in a moment what's happened."
The 29-year-old Argentine's foot appeared to get stuck on the floor and he crumpled down, landing on his left hand and bending it backwards.
He needed several minutes of medical attention and sat on a chair at the back of the court, wincing in distress.
Incredibly, he broke the serve of the unseeded Troicki on his immediate return to action.
Swiss legend Federer booked his spot as he eased to a 7-5, 6-4 victory over the unseeded Frenchman Richard Gasquet.
Federer, the second seed, has not dropped a set all week and he and old rival Nadal appear headed on a collision course.
"We all hope it's a precaution more than anything," Federer said when learning that Del Potro was off to see the injury assessed.
"For me, at the end of the day, nothing changes, I'm ready to come out here tomorrow and see the match like it's a revenge chance for the US Open," Federer added, referring to when Del Potro beat him in the quarter-finals this summer in New York.
- Gutsy Nadal battles on -
Nadal, who has never won the Shanghai Masters, is chasing a seventh title in a brilliant season and a hat-trick of triumphs on the trot, having been crowned US Open and China Open champion in recent weeks.
The 16-time Grand Slam winner saw off the Bulgarian Dimitrov in Beijing last week but suffered some hairy moments in getting the better of him again in a nervy quarter-final played in overcast conditions.
"Very happy, it was a very tough match, both of us played a very high level of tennis," said the 31-year-old Spaniard, the top seed.
"I enjoyed it, a great battle between two players that are playing all the time very focused and intense points.
"A lot of matches in a row winning and very happy with everything, let's see what happens tomorrow.
"Just focus on holding that momentum."
Nadal edged in front when he got the break of serve in the ninth game of the first set and there was nothing between him and his good friend Dimitrov as they entered a tense second-set tie break.
The Spaniard got the mini-break for a 3-0 lead in the tie break, but sixth seed scrapped back to level, and then stunned Nadal to clinch the tie break.
Dimitrov sensed another opportunity in the fifth game of the third set, but Nadal survived the break point.
Nadal took the momentum into the next game to immediately break Dimitrov and finally end the Bulgarian's resolve.
The Croatian fourth seed Cilic -- who has defeated Nadal only once in five matches -- beat unseeded Spaniard Albert Ramos-Vinolas 6-3, 6-4.