The world number five from Australia, who lifted last week's Washington Classic title and is the highest-ranked player left in the field after the withdrawal of top seed Andre Agassi, was joined in the last 16 by defending champion Paradorn Srichaphan.
"The last three matches were day matches and I haven't played here before, so it took some adjusting tonight before I got into form," Hewitt told reporters.
"I played better in the second set. I just want to sharpen up a few areas of my game before (the U.S. Open) next week."
Hewitt, who slipped from world number one at the start of 2003 to 16th by the end of last year, said a busier schedule has helped him get his confidence back.
"The biggest reason I fell in the rankings was because I didn't play in very many matches," he said.
"But I did play well, and the players I lost to were very good. So it wasn't so much a matter of me playing poorly as just not playing so often."
Thai Paradorn, bidding to become only the second man to win the title three years in succession, eased past American Kevin Kim 6-3, 6-2.
Seventh seed Thomas Johansson of Sweden also progressed but there was a surprise defeat for sixth seed Mario Ancic.
The Croat, a semi-finalist at Wimbledon earlier this year, went down 6-1, 6-4 to Russian Dmitry Tursunov.
