Phelps and Hoff lowered world marks in the men's and women's 400m IM and the American record in the men's 400m freestyle fell at the hands of Jensen.
In the men's 400m IM, Phelps edged Ryan Lochte (Daytona Beach, Fla.), 4:05.25 to 4:06.08. Both swimmers' times were faster than the previous world record of 4:06.22, set by Phelps at the 2007 FINA World Championships. They are now the top two swimmers of all time in this event.
"That was one of the most painful races ever," Phelps said. "I definitely wouldn't have been able to do it without Lochte in the pool next to me."
Phelps took a body-length lead over Lochte at the 100-meter mark, only to have Lochte chip away in the backstroke and breaststroke legs. Both swimmers were under world-record pace at 250 meters, and only two-hundredths of a second separated them as they headed into the final 100.
"You saw how excited I was after that race," Phelps said. "I couldn't start the Trials off on a better foot. I'll expect a dogfight every time we get in the water. I love racing him, and I look forward to the 200 IM."
Hoff's time of 4:31.12 in the women's 400m IM Sunday broke the former world record of 4:31.46, held by Australia's Stephanie Rice, by 34-hundredths of a second.
Hoff led for the first 100 meters, but fell behind 15-year-old Elizabeth Beisel (Saunderstown, R.I.) during the backstroke leg. She then regained the lead at the 250-meter mark and never looked back. Beisel finished second in 4:32.87, the third-fastest time in the world this year and two-hundredths of second faster than Hoff's former American record of 4:32.89.
"I was just trying for a best time, and it happened to be a world record," Hoff said. "This one was definitely a shock to me."
Jensen came out on top of an extremely competitive race, turning in a time of 3:43.53 in the men's 400m free.
The race featured a showdown between the four fastest Americans of all time in this event, including Jensen, former American record-holder Peter Vanderkaay (Rochester, Mich.) two-time Olympic bronze medalist Klete Keller (Ann Arbor, Mich.) and Erik Vendt (North Easton, Mass.).
Jensen took the lead and was under American record pace at the 200-meter mark, but was challenged by Vanderkaay and Vendt as the three swimmers battled down the homestretch. Vanderkaay finished second in 3:43.73, which was also faster than his former American record of 3:43.82. Vendt was third by a fingertip, touching in 3:43.92.
"I knew they were fast chargers," Jensen said. "I knew they were coming hard, and it was up to me to hold them off. I just buried my head and tried to get my hand on the wall first. Luckily I did."
Semifinals of the women's 100m butterfly and men's 100m breaststroke were also held Sunday. The University of Tennessee's Christine Magnuson led the field in the women's 100m fly with a time of 57.50. World record-holder Brendan Hansen (Havertown, Pa.) was just 11-hundredths off his world record in the men's 100m breast, turning in a 59.24. The top eight swimmers from tonight's semifinals will advance to the finals tomorrow night.
Tomorrow's events include prelims and semifinals of the women's 100m backstroke, the men's 200m freestyle, the women's 100m breaststroke, women's 400m free and the men's 100m back. The prelims can be viewed via live webcast at www.nbcolympics.com. Finals will be conducted in the women's 100m butterfly, men's 100m breast and women's 400m free. Finals will be broadcast live on USA Network. The start list for tomorrow's prelims can be found at www.omegatiming.com.