Adrian won his heat in 48.19 seconds while swimming in a lane next to world-record holder Cesar Cielo of Brazil. World champion James "The Missile" Magnussen was fourth quickest in qualifying.
"It felt good to be that easy in the first 50 and to have a little bit to come home," Adrian said. "Maybe I can go faster, who knows?"
Phelps already has the most gold medals of any Olympian — 14 — and eclipsed Mark Spitz's record for a single games with eight wins in Beijing four years ago.
Now, with a medal in the 200 butterfly, he can equal Soviet gymnast Larisa Latynina's record of 18 career medals. He's also competing in the ffreestyle relay.
Phelps had the morning off while his teammates qualified the U.S. first in the relay heats.
In the 100 free, Cielo, the 2008 bronze medallist , tied for 10th with Konrad Czerniak of Poland in 48.67. Cielo's time was well off his world mark of 46.91 set at the 2009 world championships during the peak of the high-tech body suit craze. Those suits have since been banned.
"It felt a little heavy, actually," Cielo said. "But hopefully tonight I'm going to bring some heat and I'll swim better to make the final."
Gideon Louw of South Africa was second-quickest in 48.29, followed by Sebastiaan Verschuren of the Netherlands in 48.37.
Magnussen of Australia advanced in 48.38. No other swimmer in a textile suit has gone faster than the 47.10 he posted at his country's trials in March.
But Magnussen's opening leg of the 4x100 freestyle relay on Sunday raised some doubt about whether he can add the Olympic title to his world championship. The Aussies were favoured but finished off the medals podium in fourth.
"I took a fair hit in the relay. I am still trying to bounce back from it," he said. "It hurt my pride as much as anything else. A lot of my competitors have never seen me lose."
Adrian swam the leadoff leg for the Americans in 47.89, going out faster than Magnussen to give the U.S. an early lead. The U.S. settled for silver after Ryan Lochte was caught on the final lap by Yannick Agnel of France.
"It wasn't that I was nervous. I just got too worked up over it," Magnussen said. "I just need to stay relaxed and be confident. They probably think they can beat me, but I am going to fight for this one."
Cielo wasn't ready to write off the Aussie.
"That was one mistake. You can't put those guys out of contention," he said. "I think James is still one of the favourites. I would put him and Agnel down as hands-down the favourites."
Adrian agreed with Cielo, saying that a full day's rest could make a difference.
"They got those first-race jitters out of them and now I would not doubt that they're a lot faster and a lot tougher in the individual," Adrian said.
Agnel tied James "The Rocket" Roberts of Australia for 12th in 48.93.
"I didn't sleep much last night," said Agnel, who won the 200 free on Monday. "I feel like an alien. I knew it would be tough to do but I have got all day to recover."
American Cullen Jones advanced to the 16-man evening semifinals in ninth at 48.61.
The other tie was for sixth between Brett Fraser of the Cayman Islands and Pieter Timmers of Belgium at 48.54.
The 4x200 relay final will feature a rematch between the Americans and the French.
The U.S. team of Charlie Houchin, Matt McLean, Davis Tarwater and Conor Dwyer had the fastest time of seven minutes 6.75 seconds in the heats — a whopping 2.43 seconds ahead of the second-place French. Jeremy Stravius, Gregory Mallet, Amaury Leveaux and Clement Lefert clocked 7:09.18.
Phelps and Ryan Lochte are likely to be on the U.S. team for the final, while the French will bring their heavy hitters, too.
Germany advanced in third and Australia was fourth.
In the men's 200 breaststroke heats, Daniel Gyurta of Hungary was fastest at 2:08.71. He won the silver at the 2004 Athens Games.
Michael Jamieson of Britain was second at 2:08.98, followed by teammate Andrew Willis in 2:09.33.
Kosuke Kitajima of Japan, trying to win the 200 for the third Olympics in a row, qualified fifth at 2:09.43. He already failed to defend his title in the 100 breast, finishing fifth.
"I am just rebuilding my motivation for the 200 after what happened in the 100," he said.
American Clark Burckle was sixth at 2:09.55, while teammate Scott Weltz was seventh in 2:09.67.
In the women's 200 butterfly heats, American Kathleen Hersey led the way in 2:06.41. The U.S. hasn't medaled in the event since 2000 when Misty Hyman stunned Australian Susie O'Neill in Sydney to win gold.
World champion Jiao Liuyang of China, the 2008 silver medallist , was second-quickest at 2:07.15. Jemma Lowe of Britain had the home fans cheering loudly when she qualified third at 2:07.64.
American Cammile Adams was eighth. Also advancing to the evening semifinals were defending Olympic champion Liu Zige in 11th; 2008 bronze medallist Jess Schipper of Australia in 12th; and 2004 gold medallist Otylia Jedrzejczak of Poland in 16th.
Ellen Gandy of Britain, the silver medallist at worlds, was 17th and missed the semis by one spot.