Kim Clijsters admitted she ran out of "fighting spirit" as she lost to Victoria Azarenka in the quarter-finals of the Sony Ericsson Open in Miami.
Defending champion and second seed Clijsters was beaten 6-3 6-3 by the eighth seed from Belarus, who is looking to repeat her 2009 title success at the WTA Tour event.
After battling from the brink of defeat to win against Ana Ivanovic in the fourth round, Belgian Clijsters could not manage another comeback against Azarenka who goes on to face Russian Vera Zvonareva in the semi-finals.
Renowned for her tenacity, Clijsters conceded it just was not there against Azarenka.
She said: "I think tonight was going to be very tough obviously. But I just didn't feel good out there. Just mentally, physically, I didn't feel right.
"It has been a tough few last days. But not in a way that I should not be ready for that. I train hard enough to physically be capable of doing that.
"Mentally I just kind of feel like I didn't have any fighting spirit. It's obviously tough against someone like her.
"Against some lower-ranked players maybe it's possible to get through matches, but against somebody like her, not."
Azarenka was able to turn her attentions to Thursday's late-night semi-final against Zvonareva.
"I have to play on a high level, because she's a very consistent player," Azarenka said.
"She's showing some of her best tennis in her life right now, so I definitely have to be aggressive and focused on my game to play forward and have fun."
Third seed Zvonareva proved too strong for ninth seed Agnieszka Radwanska as she overcame her Polish opponent 7-5 6-3.
Zvonareva said: "I'm pleased the way I played today, and especially against Agnieszka it's never easy.
"She keeps the ball in play, so you really have to focus really well.
"I was able to keep my concentration throughout the whole match. There were still ups and downs, but overall I'm really happy.
"Especially in those windy conditions, I had to take my chances and come into the net. I was pretty successful out there, so I'm pleased with that as well."