The 19-year-old, who is aiming to win six gold medals here, pulled away from Australian team mate Grant Hackett over the second half of the 400 freestyle to win in three minutes, 45.28 seconds, well outside his own world record time of 3:40.08.
Thorpe then produced an incredible finish to anchor Australia to victory in the 4x100 metres freestyle in 3:15.15 after an epic struggle with American Jason Lezak.
Thorpe had broken the 400 metres world record on the opening night of every major competition since the 1999 Pan Pacific championships, including the recent Commonwealth Games in Manchester, where he finished with six golds. But a punishing schedule over the summer dashed his hopes of continuing that remarkable sequence.
“I wasn’t trying for a world record. To get down to 3:40 would have taken a lot out of me and at this stage of the meet I didn’t want to do that,” said Thorpe, who also holds the world record in the 200 and 800 metres freestyle.
However, Thorpe brought a sell-out Yokohama crowd to life by coming back from a seemingly hopeless position after he had been overtaken by Lezak in the final race of the opening day.
Hackett had given Thorpe a 0.26 lead going into the anchor leg of the 4x100 metres relay but Lezak quickly overhauled him at the turn and powered into what looked like a winning lead halfway down the final 50 metres.
But Thorpe dug in and came back from half a body length down in the final 20 metres to hit the wall 0.26 seconds ahead of Lezak to give Australia a 4-1 lead over the US in gold medals after the first day.
Commonwealth Games champion Jennifer Reilly had already given Australia the perfect start when she won the first gold medal of the championships, taking the 400 metres individual medley title in 4:40.84.
American Maggie Bowen took silver in 4:44.39 with Japan’s Maiko Fujino a surprise bronze medallist in 4:45.79.
Diana Munz claimed the first American gold of the competition by winning the women’s 1,500 metres freestyle in 16:07.86 ahead of Sachiko Yamada in a Japanese record 16:16.28.
But Australian quartet Jodie Henry, Alice Mills, Petria Thomas and Sarah Ryan stormed back to take the women’s 4x100 metres freestyle relay in 3:39.78 with the US settling for silver in 3:40.23.
Results of finals:
MEN’S:
400 metres freestyle: 1. Ian Thorpe (Australia) three minutes 45.28 seconds; 2. Grant Hackett (Australia) 3:45.99; 3. Klete Keller (US) 3:48.40; 4. Erik Vendt (US) 3:49.75; 5. Shunichi Fujita (Japan) 3:51.40; 6. Mark Johnston (Canada) 3:52.29; 7. Naoya Sonoda (Japan) 3:55.98; 8. Bruno Bonfim (Brazil) 4:00.20.
4x100 metres freestyle relay: 1. Australia 3:15.15; 2. US 3:15.41; 3. Canada 3:17.69; 4. Brazil 3:22.89; 5. New Zealand 3:29.13; 6. Singapore 3:33.03; 7. Hong Kong 3:33.49; Japan disqualified.
WOMEN’S:
1,500m freestyle: 1. Diana Munz (US) 16 minutes 07.86 seconds; 2. Sachiko Yamada (Japan) 16:16.28; 3. Claire Hentzen (US) 16:29.25; 4. Nayara Ledoux Ribeiro (Brazil) 16:33.82; 5. Ai Shibata (Japan) 16:33.86; 6. Taryn Lencoe (Canada) 16:42.82; 7. Amanda Pascoe (Australia) 16:46.84; 8. Rebecca Linton (New Zealand) 17:18.92.
400 metres individual medley: 1. Jennifer Reilly (Australia) 4:40.84; 2. Maggie Bowen (US) 4:44.39; 3. Maiko Fujino (Japan) 4:45.79; 4. Elizabeth Warden (Canada) 4:45.80; 5. Sawami Fujita (Japan) 4:48.40; 6. Elizabeth Van Welie (New Zealand) 4:49.95; 7. Madeleine Crippen (US) 4:50.49; 8. Jessica Abbott (Australia) 4:51.59.
4x100 metres freestyle relay: 1. Australia 3:39.78; 2. US 3:40.23; 3. Japan 3:42.23; 4. Canada 3:47.18; 5. New Zealand 3:49.75; 6. Brazil 3:53.81; 7. Hong Kong 3:55.70; 8. Singapore 3:58.93.—Reuters