KAZAN, Russia -- Katie Ledecky stood on the podium, a gold medal slung around her neck. After leaving rivals in her wake in the pool, she was alone smiling on the top spot while the other medalists departed.
Seemingly uncertain what to do next and with no one around to tell her, Ledecky sheepishly hopped off and headed down the stairs to the deck.
That was the only direction the 18-year-old American lacked on the opening night of pool swimming Sunday at the world championships.
Ledecky began an ambitious program with a 3.89-second victory in the 400-meter freestyle, falling off her world-record pace on the next-to-last lap. She touched in 3 minutes, 59.13 seconds to set a championship mark and defend the title she won two years ago in Barcelona.
She set the world record of 3:58.37 at last year's Pan Pacific championships in Australia.
"When I touched I kind of thought that I had it," she said. "I realize that I'm not going to break a world record every time I get up on the blocks. But I want to be awfully darn close every time and that's what keeps me motivated."
Ledecky, a recent high school graduate who has yet to earn a driver's license, will be favored in the 800 and 1,500 freestyles and is a threat in the 200 free.
"I'm really happy with my speed in that," she said after Sunday's race. "That sets me up really well for the rest of the week."
Sharon van Rouwendaal of the Netherlands took silver in 4:03.02. She earned silver in the 10-kilometer open water race last week. Jessica Ashwood of Australia earned bronze in 4:03.34.
Sun Yang of China, who like Ledecky has four individual events at worlds, won his third consecutive world title in the men's 400 free. The current Olympic champion won in 3:42.58 and climbed the podium wearing shoes with flashing lights on the soles.
The first half of the race belonged to James Guy of Britain, who was under world-record pace while being chased by Sun, who is coming off a three-month doping ban last year after testing positive for a banned stimulant.
"For doping cases I don't understand why the media pay so much attention to this. I think it's a lack of respect," Sun said through a translator. "Around the world, when China has good results, people always think something bad. We are training as hard as all athletes in other countries. There is absolutely no doubt about our performances. There are doping cases in other countries too."
Sun pulled slightly ahead of Guy approaching the last wall and then built a bigger lead over the final lap to clinch the victory. He sat on the lane line and clenched both fists in celebration.
Sun said he put off surgery for injuries to his hand and shoulder so he could compete at the worlds.
"I've been dreaming of winning in Kazan," he said.
Guy took the silver medal in 3:43.75 and Ryan Cochrane of Canada earned bronze in 3:44.59.
"Sun Yang is a distance swimmer so he has a lot more endurance than I do," Guy said. "My best chance to beat him was to go in hard and try and hold on, and that's what I did."
Connor Jaeger of the United States, the bronze medalist two years ago in Barcelona, finished fourth in 3:44.81.
France outdueled Russia to win the men's 4x100 free relay in 3:10.74. Mehdy Metella, Florent Manaudou, Fabien Gilot and Jeremy Stravius took the lead on the third lap and hung on by 0.45 for gold.
Cheered by the near-sellout home crowd, Russia's team of Andrey Grechin, Nikita Lobintsev, Vladimir Morozov and Alexander Sukhorukov touched second in 3:11.19.
Luca Dotto, Marco Orsi, Michele Santucci and Filippo Magnini of Italy earned bronze in 3:12.53.
The United States, winners of the relay at 11 of the 15 previous world meets, surprisingly failed to make the final after winning its preliminary heat, but tying with Germany for 11th. Only the top eight make the final.
"You win your heat, usually you're in the final," U.S. national team director Frank Busch said. "Sometimes you can underestimate what your opponents are doing, so I think maybe there was a little bit of that. A hard lesson learned."
Australia won the women's 4x100 free relay in a championship-record time of 3:31.48. Emily Seebohm, Emma McKeon and sisters Bronte and Cate Campbell took the lead for good at the 200-meter mark.
Ranomi Kromowidjojo, Maud van deer Meer, Marrit Steenbergen and Femke Heemskerk of the Netherlands finished second in 3:33.67. The U.S. team of Missy Franklin, Margo Geer, Lia Neal and Simone Manuel was third at 3:34.61.
Sarah Sjoestrom of Sweden set a world record in the semifinals of the 100-butterfly. Competing in the second semi, she won in 55.74 seconds to lower the old mark of 55.98 set by American Dana Vollmer at the 2012 London Olympics. The final is Monday night at Kazan Arena.
"I've been aiming for this record for a long time so it's great to finally have it," said Sjoestrom, who is entered in four other sprint events.