Michael Phelps' run-up to his last Olympics begins with this week's United States national championships.

The 14-time Olympic gold medalist is facing his biggest month of the year in the pool, but the challenge is more mental than physical.

Phelps has competed sparingly so far this season, and hardly achieved the results he's used to. A month ago in Paris, he struggled in his freestyle events and appeared off form.

"Not being as satisfied as I want to be leaving here I think is more than likely going to happen," he said on Monday. "It's going to be a meet that is more mental than physical."

There's a lot at stake when the five-day nationals begin in Irvine, California, today. The top two finishers in each event qualify for the Pan Pacific championships later this month, and from that group, the US team for next year's world championships will be chosen.

Failure to qualify for those teams means American swimmers won't be competing at a major international meet until the 2012 London Olympics.

Phelps is scheduled to swim five events in Irvine: the 200-meter freestyle, 100 butterfly, 200 fly, 200 individual medley and 200 backstroke.

His busiest day will be his first, when he competes in two events tomorrow.

"I'm more motivated now than I have been all year," he said. "Not being where you want to be is frustrating. You know what you need to do to get back and you know what you need to change."

Phelps is still perfecting his new freestyle stroke, which was flawed in Paris, where his timing and stroke were off. "It is a lot better than it was a couple weeks ago. I feel like I can actually swim freestyle again. That's always a good feeling coming into a nationals meet."

Many of Phelps' Beijing Olympic teammates are here looking to solidify their schedules for the next two years.

Ryan Lochte injured his left groin last week, which has curtailed his breaststroke practice. He planned to test the stroke before deciding whether to swim the individual medleys, where he would compete against Phelps in the 200 IM. If he can't do breaststroke, Lochte still has an ambitious schedule planned with five other events, including the 200 backstroke against Phelps and Aaron Peirsol.

Natalie Coughlin is competing in the 100 backstroke, 100 butterfly and 100 freestyle; a resurgent Katie Hoff could swim up to five events; and Jason Lezak bids to extend his career at 34.