Hewitt, the top seed who finished 2001 atop of the ATP Rankings, will battle second seed Andre Agassi or No. 3 Andy Roddick on Sunday. The Americans clashed in the second semifinal.
While battling his illness, Hewitt was upset last month in the opening round of the Australian Open by Alberto Martin.
Gambill saved nine of 11 break points, but the American double-faulted in the 11th game of the first set and dropped his serve to open the second.
"I felt pretty much in control of the match. I still felt like he had his chances," Hewitt said. "I just basically played my game, just go out there and play clutch tennis with the right frame of mind."
Hewitt, who saved a career-high four match points on Wednesday in a come-from-behind victory over Thailand's Paradorn Srichaphan, won six titles last year, including the U.S. Open and the season-ending Tennis Masters Cup.
Gambill reached his second straight semifinal. He also got to the final four last week in Memphis after going 0-3 in his first three tournaments of 2002.