Tyson Gay overcame a slow start and aching back to run 9.98 seconds in the 100 metres preliminaries at the US championships on Thursday.
The 24-year-old, who has run a wind-assisted 9.76 seconds this season, got off the blocks slowly after a false start charged to the field.
But he said he still believed he could break Asafa Powell's world record of 9.77 seconds in Friday's final.
"Because of the false start I wanted to play it safe so my reaction wasn't that good," said Gay. "I got a slow start.
"I felt a little sluggish in the middle of my race because I didn't warm up properly, but beside that I feel confident."
Gay added that he was troubled by a bothersome back.
"After I came out of my drive phase I didn't execute too good because my lower back was feeling kind of funny. I think it needs to be adjusted, but I'll fix all that on Friday."
Collegian Walter Dix, who is expected to be Gay's chief rival, clocked 10.08 seconds to win his preliminary.
The collegiate 100 and 200 champion said the meeting, which serves as the American trials for August's world championships, would be his last of the year.
He will not run the worlds even if he made the team, he said.
"I'm going to shut it down," Dix said.
That could come as early as Friday's semi-finals.
"If I get the (college) record (9.92 seconds) next round I'm going to stop," said Dix, who earlier this year broke the collegiate 200 record with 19.69.
Twice Olympian Breaux Greer broke the US record in the men's javelin with a throw of 91.29 metres. The mark, the best in the world this year, earned Greer his eighth consecutive US title. He set the previous record of 90.71m last month.
"I don't know if an Olympic gold or world record would satisfy me, but I want to find out," said Greer.
Former London Marathon winner Deena Kastor won the women's 10,000 in a time of 31:57.00, followed by world qualifiers Kara Goucher and Katie McGregor.
Somalian-born Abdi Abdirahman took the men's 10,000 in a slow 28:13.51. He will be joined on the US world championship team by Galen Rupp and Dathan Ritzenhein.
The championships continue through Sunday with the top three finishers qualifying for the world championships provided they have met the standards.