Michael Phelps after winning the 200m individual medley at the US nationals in Maryland on Saturday. (AP)

Maryland: Michael Phelps of the US broke his own world record in the 200m individual medley on Saturday, winning in one minute and 55.94 seconds at the US championships.

In his 23rd race in 19 days, the 18-year-old lowered the old mark of 1:56.04 he set on July 25 at the world championships in Barcelona. He had smashed an unprecedented five world records there.

“Wow. That’s all I can say,” Phelps said. “I shocked myself for sure.”

It was the fourth time since June 29 that Phelps has broken the 200m individual medley world record, which until six weeks ago was 1:58.16 by Finland’s Jani Sievinen at Rome in 1994.

Phelps reached the halfway mark in 54.03 seconds — .30 of a second under his old world-record split.

“I said, I’m going out after it. I’m going out in 54 and try to hang on,” Phelps said. “I left it all in the pool.”

Phelps does not plan another competitive swim until November in a World Cup event in Melbourne. But his fantastic finish here will help bring him back into the pool to resume training next week after a brief break.

“Ending on this note fires you up a good bit,” Phelps said. “Having this as my last swim is going to get me back in the water to practice. In the past few weeks I have been through a lot.”

The victory also made Phelps the first man to capture five individual titles at a US long-course meet. All of them came in personal bests. Phelps also won the 100, 200 and 400 freestyles and the 200 backstroke.

By setting American records in the 200 and 400 freestyles and 200 individual medley and the 400 individual medley and 200 butterfly at Barcelona, Phelps became the first man to own five US marks since Mark Spitz from 1972 to 1974.

Spitz’s legacy will be Phelps’ target again in a year at the Athens Olympics when he tries to match the legend’s record haul of seven gold medals from the 1972 Munich Games.

“Athens is a year away. That’s the only focus right now,” Phelps said. “I’m not trying to be the next Mark Spitz. I’m trying to be the first Michael Phelps.”

Phelps, who swam here in events he rarely enters as a way of testing himself, will decide in September which events he will try to swim in Athens. The US Olympic qualifiers will be held next July in California.

In taking his 15th career US crown with the world record triumph, Phelps beat runner-up Kevin Clements by 3.62 seconds and then pumped his fists in joy, a more excited celebration than his usual “number one” finger wave.

“It shows he loves to race and he can get himself ready to do whatever he wants to do under pretty much any circumstances,” said Bob Bowman, Phelps’ coach, who bet his star pupil that he would shave his head for a sub-1:56 swim. “That was big motivation,” Phelps said. “Little things push me.”

Phelps took advantage of a relaxed atmosphere and gambled with a fast start, even though he does not seem to have discovered any limits.

“There are certain times and places for this,” he said. “Being at the end of the season it’s a pretty good situation. If I did it at the worlds, I don’t think it would have worked too well. It could have come back to haunt me.”