Playing his first tournament on clay this season, Hewitt struggled through the first two rounds, needing three sets to defeat German wild card Markus Hantschk and France's Arnaud Clement. Both matches lasted more than three hours and took a lot out of Hewitt.
"I just couldn't get into the match early and the slow start really hurt me today," he said. "But he's a tough player to play. He doesn't give you any kind of rhythm out there."
Gonzalez won the first five games of the match and breezed through the first set. But after Hewitt fought back and won the second set, Gonzalez breezed in the third, handing Hewitt only his third 6-0 set loss in 357 career singles matches.
"At the beginning of the third set, I told myself that I had to get on top right away and fortunately I did," Gonzalez said. "I was able to play my game using my serve and forehand to win the points."
Australian Mark Philippoussis pulled off an upset of his own, knocking off No. 3 Roger Federer of Switzerland, 6-3, 2-6, 6-3, in the third round.
Philippoussis won the first five games of the third set to take control. Federer saved match points in the seventh game and had break points in the ninth, but Philippousis held serve to advance to the quarterfinals.
"I definitely think it's one of my best matches on clay," Philippousis said. "He is one of the hardest guys on the tour at this moment and very dangerous on every surface he plays on."
In another upset, South Africa's Wayne Ferreira knocked off No. 11 Gustavo Kuerten, 7-6 (7-4), 6-4.
A three-time French Open champion, Kuerten is struggling to revive his claycourt game. He has not won a title on the surface in nearly two years.
With the elimination of Hewitt and Federer, No. 8 David Nalbandian is the highest seed remaining.
Nalbandian, who beat ninth seed Rainer Schuettler of Germany, 7-6 (7-3), 6-2, is one of four Argentines to make it to the quarterfinals.
A reigning Wimbledon finalist, Nalbandian will face Gonzalez in the quarterfinals Friday.
Agustin Calleri and Gaston Gaudio are the other Argentines in the quarterfinals. Calleri beat Mariano Zabaleta, 6-3, 6-0, and Gaudio defeated 16-year-old Rafael Nadal of Spain, 6-2, 6-2. Nadal upset second seed Carlos Moya in the second round.
"It's great to have four Argentines in the quarterfinals, and hopefully we'll all get through to the semifinals," Gaudio said.