Swiss star Roger Federer said he felt "completely beat up" following his Davis Cup doubles tie on Saturday, a day after Rafael Nadal blasted the packed tennis calendar.
The current round of Davis Cup matches began just days after the end of the US Open, prompting Nadal to complain that players' physical wellbeing was at risk.
Nadal even mentioned the possibility of a strike, and though Federer did not go that far, after losing the doubles leg of their tie against Australia he conceded that both he and partner Stanislas Wawrinka were deeply fatigued.
"Our chances to win now are extremely slim obviously, especially with Stan (Wawrinka) not feeling really well and I'm completely beat up too," Federer said after the four-set defeat in the World Group playoff in Sydney.
"We were trying to really win this doubles, that was our big goal and we failed so we are disappointed so it's going to be a tough day tomorrow."
Australia now lead the best-of-five tie 2-1 with two singles matches to play on Sunday.
Spanish star Nadal warned on Friday that the Davis Cup was at risk of losing credibility if the scheduling problem was not addressed.
"It is unacceptable that two big events like a Grand Slam and a Davis Cup semi-final are so close together. If this continues, the best players in the world will stop playing in this competition," he said.
World number one Novak Djokovic, who beat Nadal in the US Open final, pulled out of the first day of Serbia's Davis Cup semi-final against Argentina in Belgrade due to fatigue and a back injury.