Bumgarner has tightness in his rib cage and a nerve condition in his left foot. The ace left-hander said both issues were minor but he wanted to take care of them now rather than let them linger.
Heston allowed one hit, struck out four and walked none. The right-hander went 12-11 with a 3.95 ERA in 31 starts last year.
"I'm one year wiser and I know what to expect," Heston said. "I just want to put myself in a good position."
Padres starter Brandon Maurer completed three innings. The only hit he gave up was Williamson's shot. He walked one and struck out one.
"It was a fastball," Maurer said of the home run pitch. "He got a pretty good swing on it."
Padres: Maurer was so efficient that he finished his day with 15 pitches in the bullpen. He threw 31 against San Francisco.
"To be honest I was pretty efficient out of the pen," Maurer said as he continues to make the transition to starter from relieving. "I had my nerves under control this time."
Maurer made his last major league start on May 29, 2014, for Seattle. He was sent to the minors and returned a month later as a reliever.
"I'm just trying to repeat the same stuff," Maurer said of the change in roles. "My sinker was good. I got quite a few rollovers."
Giants: Heston was in the rotation for most of last year, which included a no-hitter at the New York Mets. He replaced right-hander Matt Cain after an injury. He may have to do the same thing this spring.
"I'm excited about where I am," he said. "The goal for me was to come in ready and be ready for anything. I want to make the decision hard on them."
Padres: RHP Brandon Morrow (shoulder surgery) threw live batting practice on Friday and continues a program that will allow him to pitch in a game soon.
Giants: The plan for Bumgarner is to skip two starts and still have time to build his pitch count toward opening day. "I'll probably give it a day or two," Bumgarner said. "I'm really hopeful that in just a couple of days it'll be back to normal. I feel like it's going to get better really quickly."