FEDERAL WAY, Wash. (AP) Michael Phelps didn't know if he had enough room left to win another swimming championship Friday night.
Turns out he did have enough - but barely.
Phelps surged ahead in the final 10 meters and out-touched Tom Shields by three-hundredths of a second to win the 100-meter butterfly at the U.S. Winter National Championships.
The 30-year-old Phelps, an 18-time Olympic gold medalist, finished in 51.38 seconds.
''I saw Tom at the break and I 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 Thursday, and is set to compete in the 200 butterfly Saturday.
Shields was in front and Phelps was in third place, 36 hundredths of a second behind, as they went into the 50-meter turn. Coming down the stretch, Phelps passed Santo Candorelli, but was still trailing Shields. He caught up with about 15 meters 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 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 the season really all of the last two years. Now I did it twice I one day,'' Phelps said. (He went 51.96 in Friday morning's preliminaries.) ''I can't complain too much.''
Missy Franklin came from fifth place halfway through the 100 backstroke to beat Ali Deloof by seven-tenths of a second, clocking 1:00.03. Franklin won the 2012 Olympic gold medal in the event.
Phelps wasn't the only one to win a second Winter Nationals title Friday.
Maya DiRado, Allison Schmitt and Conor Dwyer also raced to their second victories. DiRado took the women's 400 IM in 4:36.85 after winning the 200 IM on Thursday, Schmitt took the women's 200 free in 1:56.77 after taking the 400 free, and Dwyer won the men's 200 free in 1:46.62 after winning the 400 free.
Chase Kalisz won the men's 400 IM, Dana Vollmer the women's 100 butterfly, Yulia Efimova the women's 100 breaststroke, Sam Tierney the men's 100 breast, and Matt Grevers the men's 100 back.