BOSTON — Shawn Johnson was still in the corner, chalking up and waiting for the go-ahead when her floor exercise music began.

It was the closest thing to trouble she had all day.

The reigning world champion shrugged off the glitch with her music, winning her second straight title in the U.S. gymnastics championships today and taking the first round in a rivalry with teammate Nastia Liukin that is sure to captivate all the way through the Beijing Olympics.

"It definitely feels great to be in the position I am going into the trials and Beijing," said Johnson, who has lost only one meet in the last two seasons. "Overall, I feel like I had a really good meet."

Johnson's score of 127.5 was the highest at nationals since the open-ended scoring system began in 2006, and was a point better than Liukin.

For all the talk about Johnson's tough vault and Liukin's difficult uneven bars, the difference came on floor: Liukin fell in Thursday's preliminaries, scoring more than 1.5 points lower than she did Saturday.

"It's hard to think about that," Liukin said. "There are always, `What ifs?' But you can't really think about that because it is what it is."

The good thing for Liukin — and gymnastics fans — is there will be plenty of do-overs this summer.

Johnson, Liukin and 17 others now move onto the Olympic trials, which are June 19-22 in Philadelphia. The top two there earn spots in Beijing, where the U.S. women will be favored to win the gold medal. The remaining four members of the team and up to three alternates will be named after a July 20 training camp.

Comeback kid Chellsie Memmel helped her cause, finishing third with another impressive performance. The 2005 world champion has missed most of the last two years after blowing out her right shoulder, and national team coordinator Martha Karolyi admitted she'd "almost lost patience" with Memmel's slow recovery.

But she is looking more and more like the gymnast who carried the Americans at the 2003 world championships. She finished second to Liukin on uneven bars, and was fourth on both floor exercise and balance beam. Her floor routine was so expressive, it brought a big smile to Karolyi's face.

"I think I surprised a lot of people," Memmel said. "I think a lot of people wrote me off and said, `I don't think she can do it, she hasn't competed in so long, she's getting old.' It was proof to myself and to everybody else that I'm still around and I can still make contributions to the team."

Liukin and Johnson are completely different gymnasts, which is what makes watching them so entertaining. Johnson is power and precision while Liukin is grace and beauty. Johnson is one of the strongest tumblers in the world, and she makes the balance beam look 4-feet-wide, not 4 inches. Liukin has such perfect body lines, it's impossible for her to get in an ugly position, and dancers could learn a thing or two from her.

But both are very, very good.

Liukin has nine medals from the world championships, tying Shannon Miller for most by an American. Johnson won three gold medals at last year's world championships including the biggest prize, becoming only the fourth U.S. woman to win the all-around title.

So when it comes to bragging rights, something has to give with these two.

Competing in the same group, Johnson got the early edge because they started on vault. Johnson's vault is so difficult, only a handful of other women in the world even try it. When she hit the vaulting table, her hands were off to the left side and she took a step to secure her landing. But she stood up with a big grin, and was on her way with a score of 16.

"I have a lot more confidence in my vault now," said Johnson, who fell the first time she tried it in competition. "I feel like I can just get better at, work on the landings."

Next up were the uneven bars, where Liukin's routine is so packed with difficulty it leaves her gasping as if she'd just run the 200 meters. And yet she performs it with such grace and beauty, it looks almost easy. She seems to be suspended in the air as she floats from the high bar to the low bar, and every move is done with precision: pointed toes, legs that are glued together.

But all that work takes a toll on her, and Liukin ran out of gas at the end, almost landing her dismount on the knees.

She still scored a 17.1, topping the record 17.05 she got in Thursday's preliminaries. Those are the highest scores for an American — man or woman, on any event — under the new scoring system.

"Going into today, I was like, `I can't get lower than 17!"' Liukin said. "I felt it was a better routine than Thursday night, up until the dismount."

Both were dazzling on beam, tumbling and twirling high in the air as easily as if they were on flat ground.

That brought them to floor, the event where Liukin had stumbled Thursday night.

Johnson went first, and the event is the perfect showcase for her bubbly personality. She gets nosebleed-high on her tumbling passes, yet lands them so solidly she may as well be settling into quick-dry cement. She bounds around the floor with a smile on her face, looking not too different than the kid who used to stack her toys together so she could climb on them.

"I love performing my new routine, and I feel like it really fits me," said Johnson, who found her "August Rush" music herself. "It's a lot of fun. I feel like I can play to the ground and enjoy what I do."

Johnson was beaming even before she saw her score of 16.2, which put her out of Liukin's reach. Still, Liukin put on a beautiful performance of her own. Tall and lithe, she looks more like a ballerina as she dances and twirls, yet she's got the tough tricks, too.

"I wish I wouldn't have had that fall (Thursday), but it teaches you something," Liukin said. "I'm happy with how everything turned out. It's finally the summer and the first step is under our belt."