Sean Rooney added 12 points in the 25-15, 25-19, 25-19 win, which set up a quarterfinal against Italy on Wednesday. Tunisia finished the competition at Earls Court without a win.
The United States was coming off a five-set loss to Russia, which ended an 11-match winning streak in Olympic play, dating to the Americans' undefeated march to the gold medal in the Beijing Games.
The Americans' preliminary round was highlighted by a four-set victory over top-ranked Brazil, a rematch of the Beijing final.
The fifth-ranked U.S. men weren't considered among the medal favourites at the London Games, but they claimed silver in the recent World League tournament before opening the Olympics with dominant straight-set victories over Serbia and Germany.
Against No. 20 Tunisia, coach Alan Knipe rested veterans Clay Stanley, Reid Priddy and David Lee. The team didn't need them, claiming the first set on Matt Anderson's spike and cruising the rest of the way. David Smith spiked for match point.
"We have some guys that haven't played much that are extremely talented volleyball players. On one side they're fresh so you knew they were going to be full of energy and really excited about getting their opportunity," Knipe said. "And on the other side of that we got some guys who have played a lot of sets.
"So we have important matches hopefully coming up. We wanted to manage both and I think we accomplished that tonight."
Stanley, a three-time Olympian who was named MVP of the Beijing Games, appreciated the break before the knockout round.
"It's nice when you have 12 guys who can all get the job done," he said.
The top four teams in each pool, based on a point system, advanced to play in the quarterfinals. The top finishers in each pool face the fourth-place finishers in the opposite pool, while the other matches were determined by drawing lots.
In addition to the U.S. match against Italy, Bulgaria plays Germany, Poland faces Russia and Argentina plays Brazil.
Tsvetan Sokolov scored 21 points in Bulgaria's straight-set victory over Italy to secure the top seed in its pool.
Todor Aleksiev added 13 in the 32-30, 25-20, 25-19 victory for the surprisingly strong Bulgarians, who dropped only one of five preliminary round matches.
Ivan Zaytsev had 15 points for Italy, which finished the opening round 3-2.
The ninth-ranked Bulgarians came to London in disarray. After the team earned an Olympic berth, the coach and a top player resigned to protest their country's leadership of the sport. Nine of 12 players are first-time Olympians.
"We have created this really good climate among all the players," coach Nayden Naydenov said. "That's why we are playing so well. They are a really good team."
In the biggest upset Monday, No. 22 Australia beat No. 3 Poland 3-1 (25-21, 25-22, 18-25, 25-22). Travis Passier's ace proved the match-winning point, and the Volleyroos earned a standing ovation from the Earls Court crowd.
The loss kept Poland from a top seed.
"We lost control in the beginning, and we were not really fighting," Poland coach Andrea Anastasi said. "I have no explanation."
The Volleyroos, making their first-ever appearance in Olympic volleyball, needed winless Britain to defeat Argentina in an afternoon match to advance. But the eighth-ranked Argentineans defeated the hosts 25-18, 25-18, 25-15.
Russia eliminated Serbia 25-15, 25-20, 25-17, and Brazil wrapped up the night with a 25-21, 25-22, 25-19 victory over Germany.