Michael Phelps opens his quest for an unprecedented eight Olympic gold medals today in an event which might turn out to be his toughest challenge.
Phelps has long been master of the 400 metres individual medley but, with fellow American Ryan Lochte edging ever closer, he acknowledges it will be one of the hardest races to win as he embarks on a schedule aimed at 17 starts in nine days.
"They're all going to be hard. With so many races I think one of the hardest will be one of the first, the 400 IM," he said this week as he looks to exceed the seven golds won by Mark Spitz in 1972.
Phelps lowered the 400 medley world record for the seventh time at the US trials in June but Lochte was less than a second adrift and also inside the old world mark.
Lochte, twice a silver medallist behind Phelps at the 2007 world championships, would dearly love to beat him for once.
"I wouldn't mind at all. I wish him all the best, he is my friend, but if he doesn't do it, it means I did something right," Lochte said.
Grant Hackett, whose big goal is a unique third Olympic 1,500 metres freestyle title, enters the evening heats action in the Water Cube among the favourites to succeed fellow Australian Ian Thorpe as Olympic 400 freestyle champion.
Hackett, silver medallist behind the now-retired Thorpe in 2004, holds this year's fastest time but faces tough competition from Park Tae-hwan, South Korea's first world champion, Americans Larsen Jensen and Peter Vanderkaay and US-based Tunisian Oussama Mellouli, back from suspension having lost world gold and silver medals after testing positive for an amphetamine.
Australian team-mate Libby Trickett opens her bid to match Inge de Bruijn's golden treble of the 2000 Sydney Olympics in heats for the women's 100 butterfly, with the 50 and 100 freestyle to follow, plus relays.
Japan's Kosuke Kitajima, double Olympic champion in 2004, also features on the opening night as he begins the defence of his 100 breaststroke crown.
Brendan Hansen, the 2004 silver medallist who turned the tables on Kitajima at last year's world championships, will be doubly motivated after failing to qualify for the 200 at the US trials.
Kitajima, though, is in fighting mood, buoyed by a 200 breaststroke world mark in June. "I intend to break world records," he said on Thursday. "I'm confident of that.
"I'm really looking forward to racing Hansen."
World champion Katie Hoff, tackling a marathon programme to rival that of Phelps, starts favourite in the women's 400 medley to succeed Ukraine's Yana Klochkova, who won both Olympic 200 and 400 medleys in 2000 and 2004 but has since failed to rediscover that form and is swimming only relays in Beijing.