In Olympic history, the 100 metres final has only twice gone ahead without an American swimmer -- in 1980 when the United States boycotted the Moscow Games and in 1896 when only two men, a Hungarian and an Austrian, entered.

Lezak finished fifth in his heat in 49.87 seconds, way behind Dutch defending champion Pieter van den Hoogenband, who led the way into the evening's semi-finals in 48.70.

Crocker, the world 100 metres butterfly champion, was initially listed as 16th equal overall, which would have meant a swim-off for the last semi-final place.

But Trinidad's George Bovell, initially disqualified for an infringement at the start, was reinstated on appeal and moved back in as 12th overall, which pushed Crocker and Yuriy Yegoshin of Ukraine down to 17th equal and out. Lezak moved down from 20th to 21st.

Crocker had been suffering from a sore throat on Sunday when he swam way below form in Sunday's 4x100 freestyle relay in which the American favourites trailed in third behind South Africa and the Netherlands.

Ian Thorpe, two Athens gold medals already in his possession, opened his bid for an unprecedented Olympic freestyle treble when he cruised into the 100 semi-finals.

Thorpe, who retained his 400 freestyle title on Saturday and won the 200 freestyle on Monday, did just enough to win his heat as he conserved his energy embarking on the most difficult phase of his treble chance.

The 21-year-old Australian, bronze medallist in last year's world championships, was seventh at the turn but touched first on the return and clocked 49.17, the sixth-fastest overall time.

Lithuania's Rolandas Gimbutis and South Africa's Ryk Neethling were joint second overall in 48.85.

Alexander Popov, contesting his fourth and last Olympics at the age of 32, also qualified comfortably, finishing second in his heat behind Frenchman Romain Barnier and eighth overall in 49.51.

Popov, who regained his world 50 and 100 metres freestyle titles in Barcelona last year, completed a unique 'double double' in the two sprint events at the 1992 and 1996 Olympics but had to settle for silver in the 100 behind Van den Hoogenband in Sydney, where he finished out of the medals in the 50.

Poland's Otylia Jedrzejczak, two silver medals to her credit in Athens, underlined her ambitions for gold with the fastest heat time in the women's 200 metres butterfly. World record-holder Jedrzejczak clocked two minutes 9.64 seconds to lead the way into the semi-finals ahead of Japan's Yuko Nakanishi (2:10.04).

Australia's Petria Thomas, who won Sunday's 100 metres butterfly ahead of Jedrzejczak, won her heat in 2:10.87, fifth overall.