Gwangju - China's controversial champion Sun Yang topped qualifying in the 400m freestyle at the world swimming championships on Sunday to set up a final that will be bubbling with hostility.
American stars Caeleb Dressel and Katie Ledecky also looked in peak condition on the first day of the pool competition in Gwangju, while Britain's Adam Peaty limbered up for an assault on his own 100m breaststroke world record.
Triple Olympic champion Sun shrugged off fresh claims from a leaked FINA doping panel report that he smashed blood samples with a hammer when visited by testers last year, easing home in his heat in a time of 3:44.10.
Australian rival Mack Horton, who labelled Sun a "drug cheat" over a previous three-month suspension before pipping the Chinese giant to gold in the 2016 Rio Olympic final, could only manage fifth fastest, well over a second slower.
"The first race is always difficult," Sun told reporters.
"My goal this morning was just to make sure I did what I needed to make the final and give myself some confidence. I've trained so hard this year and want to show my best in every race."
World record holder Ledecky qualified fastest for the women's 400m freestyle final in 4:01.84 with Australian teen Ariarne Titmus, the Commonwealth champion, second fastest in 4:02.42.
"I just wanted to get the first one out of the way," said Ledecky, a five-time Olympic gold medallist with 14 world titles to her name.
"I'm right where I wanted to be so feeling good moving forward."
Dressel, who swept to seven gold medals at the 2017 world championships in Budapest, topped the men's heats in the 50m butterfly with Ukraine's world record holder Andrii Govorov in a time of 22.84.
Peaty turned a fraction under world record pace in the 100m breaststroke heats but opted to conserve energy over the last 25m to clock 57.59 - over a second faster than Belarusian Ilya Shymanovich.
"I just felt good, very similar to Olympics weirdly," said Peaty, who could target his world best of 57.10 in the evening's semi-final.
"I can't go out there hunting it," added Britain's Olympic king, referring to break the magic 57-second mark.
"If it comes to me naturally down that first 50 and I put the pressure on that back end it's going to come."
Sweden's world and Olympic champion Sarah Sjostrom looked in total control as she chases a record fifth world title in the women's 100m butterfly, qualifying fastest in 56.45 with Australian Emma McKeon almost half a second back.
"To be on the top level for that long is very cool," said Sjostrom, asked about the strong possibility of becoming the first female swimmer to capture the same world title five times.
"I will just continue to try to improve."
Elsewhere, Hungary's "iron lady" Katinka Hosszu topped qualifying in the women's 200m individual medley, the world and Olympic champ clocking 2:07.02 - more than two seconds faster than China's Ye Shiwen.