Belgium's fourth seed Kim Clijsters crashed out of the Sony Ericsson Open with a 4-6 6-4 6-2 loss to China's Li Na in Miami on Monday.
Clijsters said she was struggling with a back problem
Li, ranked 17th, beat the 2005 champion and now meets Anna Chakvetadze, who beat Italy's Maria Santangelo 6-1 7-5.
Another Belgian, world number one and second seed Justine Henin, fared better beating Vera Zvonareva 6-3 6-2.
Serena Williams beat Lucie Safarova 6-3 6-4 in a rain-delayed match in which she was racially abused by a fan.
"I was shocked. I couldn't believe it. I had to do a double-take," she said. "I think I hit a double fault at that point.
"Who says these things outside of first grade? At a professional venue you don't do that. It was shocking, I could not believe he'd stoop to that level.
"I really should have said something sooner. Even the people out there were pointing to who he was. The guy was saying things that should not have been said. It was derogatory.
"Then every time I missed a shot or serve, he would say, 'That's the way to do it'."
Williams finally complained to the chair umpire and the spectator was removed by security staff and given to the police.
The Australian Open champion now faces Maria Sharapova in a rematch of their Melbourne Grand Slam showdown in January.
"This is a new day, this is a new match," Williams said of the looming contest. "It's definitely going to be a good match."
I'm physically just not capable of playing a lot of matches in a row anymore
Henin meets Russian sixth seed Nadia Petrova next and said after her match: "I had an opportunity to raise my level and that's what I did.
"The way the match happened, I'm very happy to win in two sets like that."
Petrova had fought back to eliminate compatriot Dinara Safina 3-6 6-2 6-4.
Clijsters, whose only Grand Slam singles triumph came at the 2005 US Open, said she was struggling with a back injury.
"I just have to kind of focus, make sure that I have my practice time at home and then try to peak at tournaments," she said.
"It's not easy because I'm physically just not capable of playing a lot of matches anymore in a row."
Israel's Shahar Peer battled her way into the quarter-finals with a 6-4 4-6 6-3 win over Russian third seed Svetlana Kuznetsova.