Michael Phelps will match his most grueling program since coming out of retirement, entering four events at next week’s U.S. national championships.

The winningest athlete in Olympic history will compete in the 100-meter freestyle, 100 butterfly, 100 backstroke and 200 individual medley at the meet that begins Wednesday in Irvine, California.

"I feel good," Phelps told The Associated Press on Thursday. "Just staying relaxed is the biggest thing, not putting too much pressure on myself. I’m just going out and continuing what I’ve been doing throughout the year — just having fun. That’s how I’ve been able to swim my best."

The national championships, along with the Pan Pacific Championships in Australia a couple of weeks later, will determine the American team for next year’s world championships.

Phelps retired after the London Olympics, having won a record 18 gold medals and 22 medals overall. He had accomplished all his goals and said he had no desire to swim into his 30s.

But he began plotting his comeback less than a year later, and began serious training last September.

Since announcing a return to competition in April, the 29-year-old Phelps has competed in four meets. The only other time he took on four events was the Santa Clara Grand Prix in June, when he tied for first in the 100 fly, finished second in both the 100 and 200 free, and settled for third in the 200 IM.

The nationals will give him another chance to gauge just how far he has come in a relatively short period of time — with much more at stake. His longtime coach, Bob Bowman, said Phelps has shaken off much of the rustiness in his technique, improving his starts and turns. Conditioning is the biggest question mark, especially as he takes on more events.

Phelps is just as concerned with his mental approach.

"A couple of races, I’ve tensed up a little bit. I tried to take it super seriously and I didn’t have good success in those races," he said. "Whatever happens, happens. There’s going to be more heats, more training, where I can correct things if they don’t go as planned next week."

Phelps is set to face his biggest rival, Ryan Lochte, in all four events at Irvine.

Lochte, coming off major knee surgery, has also entered the 200 free and 200 back.

On the women’s side, 17-year-old Katie Ledecky put her name in a staggering eight events — every freestyle from the 50 sprint to the 1,500 marathon, as well as both the 200 and 400 IM. She’ll take on fellow Olympic gold medalists Missy Franklin and Allison Schmitt in both the 100 and 200 free, as well as Schmitt in the 400 free.

Franklin, one of the biggest stars of the London Games with four golds and a bronze, plans to swim four events at Irvine. She also entered the 100 and 200 back.