Nadal and Federer pulled out within two hours of each other
Top seed Rafael Nadal and second seed Roger Federer pulled out of the Paris Masters with injuries on Friday.
Federer had been due to face James Blake in the quarter-finals but withdrew because of a back injury.
And Nadal pulled out with a knee injury when trailing Nikolay Davydenko 6-1, having earlier required treatment.
In the remaining quarter-finals, Jo-Wilfried Tsonga beat Andy Roddick 5-7 6-4 7-6 (7-5), and David Nalbandian saw off Andy Murray 7-6 (7-3) 6-3.
Nadal had the trainer massaging his right thigh at the change of ends when trailing Davydenko 4-1 in the opening set, and called it quits two games later.
"I had some pain in the knee yesterday," said the Spaniard. "I felt the pain a lot when I woke up this morning.
This is the first time ever I've pulled out during a tournament, so it feels bad
"When I push (on my right knee) I feel a sharp pain."
And Nadal admitted he is worried about whether he will be fit for the season-ending Masters Cup in Shanghai, which begins on 9 November.
"For sure," he said. "Hopefully it will be fine.... I don't know."
Federer has withdrawn during a tournament for the first time in his career.
"My back has been stiff for the last couple of days and I woke up this morning and it did not respond to the treatment I had last night," he told his website.
"As a precaution and because I would be unable to play at 100% tonight, I have unfortunately had to take the difficult decision not to play."
He added: "This is the first time ever I've pulled out during a tournament, so it feels bad. I don't like to do it (and) feel bad for the tournament and for the fans."
Federer said he hopes the injury will only take a few days to heal and that he will be able to play in the Masters Cup.
"Usually the back is something that takes a few days and it's better again," said the Swiss.
"I'm confident and I'm positive that hopefully it's going to get better in the next few days."
Blake now advances to the last four where he will face Tsonga, while Davydenko will play Nalbandian.