The U.S. still looked far from gold-medal form, but it did enough against a dangerous Japan side to pick up a much-needed three points in Group G after dropping its opener against Norway.
"I am very happy about our performance today against I would say a technical and good Japan," said U.S. coach Pia Sundhage. "I think the more minutes we get together in the Olympics ... the better it is, so we will improve our game from one game to another."
And there is still room to improve. The Americans created a slew of scoring chances but were only able to capitalize on one, in the 27th minute, when Lloyd volleyed a cross from defender Stephanie Cox under the crossbar.
The 26-year-old Lloyd pumped her fists as she dashed to the U.S. bench and into the arms of her celebrating teammates.
The United States, which plays New Zealand in its final group match on Tuesday, has three points in Group G. Japan has one point and faces Norway next.
The U.S. looked determined not to repeat the flat performance that saw it surrender two early goals against Norway.
In the opening minutes, forward Amy Rodriguez, who was active up front in her first start at the Games, headed wide of the post, before Lindsay Tarpley plowed through a crowd of defenders, only to hit her left-footed shot right at Japan keeper Miho Fukumoto.
But Japan, which has never beaten the U.S. in 21 matches, threatened in the 14th minute. Forward Shinobu Ohno collected a cross in the middle of the box and dropped the ball to an unmarked Yukari Kinga, who knocked a right-footed shot just past the far post.
Japan had another solid scoring opportunity 8 minutes later, when Yuki Nagasato flicked a throw-in from Aya Miyama to Homare Sawa, whose right-footed shot forced U.S. keeper Hope Solo to palm the ball over the crossbar.
Rodriguez and midfielder Heather O'Reilly challenged Japan's back line, making a host of runs down the left and middle, and Sundhage praised their performance.
"Besides a great goal by Carli Lloyd, I'm very happy about our tactics with fast play up top -- Amy Rodriguez and the way Heather O'Reilly played on the right side," she said.
Japan coach Nono Sasaki brought on two extra forwards in the second half, but his team couldn't find an equalizer.