Hamilton admitted he was suffering by the time he crossed the line

The CSC team leader took the lead four kilometres from the finish of and finished 12 seconds ahead of Spanish rider Iban Mayo with Dutchman Michael Boogerd third.

Hamilton is a former team-mate of his more famous compatriot Lance Armstrong, who has twice been runner-up in the 258km Liege race.

Armstrong, the 31-year-old four-time Tour de France winner, had targeted winning the classic as part of his build-up to this year's Tour.

But after breaking away in the latter stages, he was caught just before the last of the day's 10 climbs, and eventually finishing 50 seconds behind Hamilton.

Hamilton's win moves him up into fourth place in the overall World Cup standings on 100 points,

"I thought someone was going to catch me. The last two kilometres were probably the hardest two kilometres in my life. I was suffering a lot," said the 30-year-old.

"To be the first American to win here is just incredible. It's not quite sinking in yet. But it's great to have won what I believe is the hardest race in the World Cup."

Hamilton also paid tribute to his former leader.

"In the years I spent at US Postal I learned a lot from Lance Armstrong, and I helped him win three Tours de France so if it wasn't for him I don't think I'd be here today," he added.

World Cup leader, Peter Van Petegem, abandoned the race after the second feeding stage.

"This isn't my kind of race, but I have maintained the lead which was my main objective and I will come back in August for the summer classics," said the Belgian.

Result of the Liege-Bastogne-Liege classic, the fifth leg of the 2003 World Cup:

1 Tyler Hamilton (US) 6 hours 28.50 minutes.
2 Iban Mayo (Spain) 12 seconds behind
3 Michael Boogerd (Netherlands) 14.
4 Michele Scarponi (Italy) 21.
5 Francesco Casagrande (Italy) 29.
6 Samuel Sanchez (Spain) 29.
7 Javier Pascual (Spain) 29.
8 Danilo Di Luca (Italy) 29.
9 Eddy Mazzoleni (Italy) 29.
10 Ivan Basso (Italy) 29.

Overall World Cup standings (after five of the 10 events)

1 Peter van Petegem (Belgium) 200 points.
2 Michael Boogerd (Netherlands) 140.
3 Dario Pieri (Italy) 117.
4 Tyler Hamilton (US) 100.
4= Paolo Bettini (Italy) 100.
4= Alexander Vinokourov (Kazakhstan) 100.
7 Frank Vandenbroucke (Belgium) 92.