Michael Phelps won a record-equalling sixth gold medal at the World championships on Saturday to remain on course for his historic bid to win eight titles.

The 21-year-old beat his American team mate Ian Crocker in a frantic finish to win the 100 metres butterfly final in 50.77 seconds, putting himself within reach of the once-unimaginable feat of winning eight gold medals at a single championships.

The only other man to win six titles at a single world championships was Australia's Ian Thorpe, at Fukuoka, Japan, in 2001. American Mark Spitz won seven golds at the 1972 Munich Olympics.

Phelps is an overwhelming favourite to complete his quest on Sunday's final night of competition by winning the 400 individual medley and helping the U.S. win the 4x100 medley relay.

"Honestly I thought I'd botched the finish, I didnt think the finish was very good, but it ended up being good enough," Phelps told reporters.

"So far we're six for six and we're definitely headed where we want to go. But tomorrow's a hard day I've got to be ready to get up and get in the finals and then have two solid races."

The 100 butterfly was always regarded as Phelps's most difficult race with Crocker having won the past two world titles and holding the world record.

Crocker led the race throughout and looked the likely winner as they approached the finish but, just as he did at the 2004 Athens Olympics, he mistimed his lunge and had to take one last stroke, allowing Phelps to get his hands on the wall first.

Crocker had to settle for the silver in 50.82 with Albert Subirats of Venezuela grabbing the bronze in 51.82.

OUTSTANDING SWIMMER

Libby Lenton enhanced her claims as the outstanding female swimmer of the championships by anchoring Australia to a world record in the 4x100 medley relay to collect her fourth gold medal.

Lenton teamed up with 14-year-old backstroker Emily Seebohm, breaststroke world record holder Leisel Jones and 200 butterfly champion Jessicah Schipper to win in 3:58.31, slicing 0.56 off the mark Australia set at last year's Commonwealth Games in Melbourne.

American teenager Kate Ziegler beat Laure Manaudou of France in a last-lap sprint to win the women's 800 freestyle final and complete the long-stance double for the second time.

The 18-year-old Ziegler trailed Manaudou when the pair turned for home but surged past her in the last couple of strokes to win in 8:18.52, denying Manaudou the chance to be become the first woman to win the 200-400-800 treble at the same championship.

The U.S. won four of the six finals on Saturday to extend their lead at the top of the medal standings.

Margaret Hoelzer upset Zimbabwe's world and Olympic champion Kirsty Coventry to win the women's 200 backstroke final in 2:07.16, breaking the championship record set by China's He Cihong in Rome in 1994.

Ben Wildman-Tobriner led an American one-two in the men's 50 freestyle sprint, slashing a quarter of a second off his personal best to win in 21.88 just in front of Cullen Jones.

Sweden's Therese Alshammar broke through to win the women's 50 butterfly in 25.91 and capture her first world title in her fifth appearance at the championships, edging out Australia's defending champion Danni Miatke and Inge Dekker of the Netherlands.