The four-time series champion kept the pressure on over the remaining laps but settled for his fifth and another solid day.
"It was OK," a slightly frustrated Dixon said after the race. "I am a little disappointed with where we started. If we had qualified better we would have been in a better position.
"Congratulations to Joseph – he did a great job and so did Ryan.
"It is fun to make another podium."
Dixon admitted he had one shot at getting past Hunter-Reay after his last stop but both drivers got on the push to pass and it proved too challenging.
"We pitted early and we had the unfortunate part of the under-cut and didn't work for us," Dixon explained.
"I had a shot there but got a little loose and that was the only real chance I had."
Newgarden has gone through a tough patch to fall out of the top spots on the ladder heading to Road America but produced a superb drive to get to within 50 points of Dixon.
"I wanted to win here since last year," Newgarden said. "This car has been a rocket all weekend. It wasn't easy – Ryan was really fast and I knew Dixon was there.
"I'm just taking each race at a time and not worrying about the championship. There is a long way to go and if we do our job each race the championship will take care of itself."
Dixon's title rivals Will Power and Alexander Rossi had bad days – Power failed to finish with a mechanical failure while Rossi finished a lowly 16th.
The next race is in Iowa in a fortnight.
