It was just the third time in 15 meetings that Merritt had beaten his rival.
"He's a great runner," Wariner said. "He's going to be my main competition for the next few years, and I'm looking forward to that because it will make me run faster."
Wariner wants to become the first to crack 43 seconds. His personal best is 43.45, the third-fastest time ever.
"One thing that's driving me at this moment is I want to defend my title at the Olympics," Wariner said. "But at the same time, the world record is on my mind. I want to be the first to run 42 seconds. Those are the things that motivate me."
Merritt burst into the limelight at the 2007 World Championships when he became the ninth man to break 44 seconds at the distance - 43.96.
Richards rolls. Sanya Richards, a three-time national champion who had been hobbled by a rare illness called Behcet's syndrome, easily won the women's 400-meter final.
Richards was never challenged, pulling away in the stretch to win in 49.89 seconds.
"I've gotten some of the nicest letters from people who suffer from that disease," she said of the disorder, which causes sores. "Most of the time they say how much I've inspired them. But their letters inspire me more."
Richards will be joined in Beijing by runner-up Mary Wineberg (50.85) and third-place finisher Dee Dee Trotter (50.88).
Three to the semis. All three Philadelphia-area runners advanced in the women's 1,500-meter preliminaries.
Erin Donohue, Carrie Tollefson and Frances Koons will run again in tomorrow night's semifinals.
Donohue, 25, a 2001 graduate of Haddonfield High School, finished second to Shannon Rowbury (4 minutes, 16.13 seconds) in her heat in 4:16.84.
"I felt really comfortable," Donohue said. "I tried to run as comfortable as possible for 1,200. I was able to tell what kind of position I was in. I adjusted from there and finished strong the last 100 meters."