Amanda Beard and Diana Munz also struck gold in Yokohama to bring the US level with Australia in the gold medal standings at five each after seeing their fierce rivals win four out of five finals Saturday.
Grant Hackett, clear favourite in the 800 metres freestyle in the absence of world record holder Ian Thorpe, cruised to victory in the last final of the evening to set up an intriguing battle over the remaining four days of the competition.
Phelps missed out on a world record but still won gold in the 400 metres individual medley, clocking four minutes, 12.48 seconds after a tight finish with American team mate Erik Vendt, who touched in 4:13.15. Japanese swimmer Takahiro Mori took the bronze in 4:16.35.
The 17-year-old Phelps was 0.43 seconds inside his own world mark after 200 metres but was caught by Vendt on the breaststroke leg and needed amazing powers of recovery to come back over the final leg of freestyle.
Phelps, who set a world record of 4:11.09 at the US nationals earlier this month, settled for a new championship best and will now turn his sights to breaking his own world mark in the 200 metres butterfly over the next two days.
Coughlin, who became the first woman to break the one-minute barrier in the 100 metres backstroke at the US nationals in Fort Lauderdale, tied a Pan Pacific record as she won the 100 metres butterfly in 57.88.
Australian Petria Thomas, who won five golds at the Commonwealth Games, had to settle for silver this time in 58.11 and eight times Olympic gold medallist Jennifer Thompson took bronze for the US in 58.64.
Beard, meanwhile, won the 100 metres breaststroke in 1:08.22 — her best time since 1996 — with US team mate Tara Kirk taking silver in 1:08.66 and China’s Luo Xuejuan third in 1:08.70.
Munz, who won gold in the 1,500 metres freestyle on Saturday, then edged out team mate Lindsay Benko to add the 400 metres title in 4:09.50 to give the US a 5-4 lead in the gold medal count — at least for an hour.
Hackett stopped the rot as he comfortably took gold in the 800 metres freestyle, clocking a championship best of 7:44.78 but well outside the world record of 7:39.16 which Thorpe set at last year’s world championships in Fukuoka.
Australia beat the US 13-9 in the gold medal standings in Fukuoka but have not forgotten the 2000 Sydney Olympics, when they got off to a flying start on day one only for the Americans to finish with 14 golds to five for the host nation.
Kosuke Kitajima briefly broke up the supremacy of the US and Australia midway through the evening session as he won the first Japanese gold medal of the championships, taking the 100 metres breaststroke title in a time of 1:00.36.
Results of finals:
MEN’S:
400 metres individual medley: 1. Michael Phelps (US) 4:12.48; 2. Erik Vendt (US) 4:13.15; 3. Takahiro Mori (Japan) 4:16.35; 4. Brian Johns (Canada) 4:16.44; 5. Shinya Taniguchi (Japan) 4:17.62; 6. Justin Norris (Australia) 4:20.19; 7. Dean Kent (New Zealand) 4:20.79; 8. Trent Steed (Australia) 4:21.45.
100 metres breaststroke: 1. Kosuke Kitajima (Japan) 1:00.36; 2. Brendan Hansen (US) 1:00.84; 3. Jim Piper (Australia) 1:01.68; 4. Mark Gangloff (US) 1:02.02; 5. Mike Brown (Canada) 1:02.21; 6. Morgan Knabe (Canada) 1:02.28; 7. Regan Harrison (Australia) 1:02.53; 8. Yoshihisa Yamaguchi (Japan) 1:02.93.
800 metres freestyle: 1. Grant Hackett (Australia) 7:44.78; 2. Larsen Jensen (US) 7:52.05; 3. Chris Thompson (US) 7:56.69; 4. Craig Stevens (Australia) 7:56.86; 5. Shunichi Fujita (Japan) 8:01.97; 6. Takeshi Matsuda (Japan) 8:11.22; 7. Kurtis Macgillivary (Canada) 8:12.27; 8. Bruno Bonfim (Brazil) 8:27.10; 9. Kwok Leung Chung (Hong Kong) 8:36.77.
WOMEN’S:
100 metres butterfly: 1. Natalie Coughlin (US) 57.88 seconds; 2. Petria Thomas (Australia) 58.11; 3. Jennifer Thompson (US) 58.64; 4. Jennifer Button (Canada) 59.68; 5. Yuko Nakanishi (Japan) 59.77; 6. Audrey Lacroix (Canada) 1:00.31; 7. Rachel Coffee (Australia) 1:00.71; 8. Maki Mita (Japan) 1:00.89.
100 metres breaststroke: 1. Amanda Beard (US) 1:08.22; 2. Tara Kirk (US) 1:08.66; 3. Luo Xuejuan (China) 1:08.70; 4. Leisel Jones (Australia) 1:08.76; 5. Brooke Hanson (Australia) 1:09.56; 6. Rhiannon Leier (Canada) 1:09.84; 7. Fumiko Kawanabe (Japan) 1:10.15; 8. Junko Isoda (Japan) 1:10.44.
400 metres freestyle: 1. Diana Munz (US) 4:09.50; 2. Lindsay Benko (US) 4:10.28; 3. Sachiko Yamada (Japan) 4:10.79; 4. Elka Graham (Australia) 4:12.42; 5. Amanda Pascoe (Australia) 4:13.33; 6. Ai Shibata (Japan) 4:13.34; 7. Monique Andrade Ferreira (Brazil) 4:13.48; 8. Jessica Deglau (Canada) 4:19.36.—Reuters