Michael Phelps Rallies to Win Another Title at US Winter National Championships

Michael Phelps surged ahead in the final 10 metres and out-touched Tom Shields by three-hundredths of a second to win the 100-metre butterfly event at the US Winter National Championships. (Swimming Icon Michael Phelps Aiming High for Rio Olympics)

The 30-year-old Phelps, an 18-time Olympic gold medallist, finished in 51.38 seconds. (Michael Phelps Swimming School Launched in India)

"I saw Tom at the break and I was like, 'Uh-oh,'" Phelps said. "But I did watch his stroke this morning, and his stroke changed a lot. It was higher tempo-based than it was in the past. So I thought if I was anywhere close in the last 25, I thought I had a chance. That's usually where I kind of accelerate a little bit."

Phelps won the 200 individual medley on Thursday, and is set to compete in the 200-metre butterfly on Saturday. (Michael Phelps Wins 200m Medley in US Winter Swimming)

Shields was in front and Phelps was in third place, 36 hundredths of a second behind, as they went into the 50-metre turn. Coming down the stretch, Phelps passed Santo Candorelli, but was still trailing Shields.

He caught up with about 15 metres left, then nudged in front just in time.

"I was thinking about the race more than I should have been," Shields said. "Maybe I took one too many looks, or I just didn't get my finishing strokes. On my way out, I swam with five different rhythms, trying to find one that works. But for right now, that's good. This isn't a trials final, it's not the Olympic final. So there's time to work through those things."

Phelps' world record 100-metre fly time, which has stood since 2009, is 49.82. He wasn't close to that on Friday, but still clearly was pleased with what he did.

"I've never been under 52 really all of the last two years. Now I did it twice in one day," Phelps said. (He went 51.96 in Friday morning's preliminaries.) "I can't complain too much."