By Julian Linden

LONDON (Reuters) - The sporting world will get their last look at Michael Phelps in action on Saturday when the greatest Olympic swimmer of all time competes in his final race before retiring.

Barring a monumental upset, the American will end his incredible career with another Olympic gold medal, his 18th, in the medley relay in London.

The U.S. have never been beaten in the event and boast a powerful lineup that includes the individual champions in three of the four strokes.

While the race is unlikely to produce much of a contest, the fact that it is Phelps' last race before he hangs up his goggles elevates it to an unparalleled status.

"The 400 medley has been such a big race for our country. We've been able to be so successful in that race and its something that we as a team look forward to," Phelps said.

"We have something in mind on how we want to finish this relay and how we want to finish this meet and hopefully we can go out and do that tomorrow."

The U.S. have dominated the swimming events this week, winning 14 of the 28 finals decided, and are also heavily favored to win the women's medley relay on Saturday.

A win would provide Colorado teenager Missy Franklin with a fourth gold medal in her first Olympics, ensuring she will take over from Phelps as the new face of American swimming.

Unlike the men, the women can expect a real challenge from Australia, who have won the event at the last two Olympics and set the fastest qualifying time.

"Australia has a great history in relays and we always have so much fun," said backstroker Emily Seebohm. "I am looking forward to the final."

China's Sun Yang will be bidding to become the first man in 32 years to complete the long-distance double when he tackles the 1500 freestyle, the longest event in the pool.

Sun won the 400 gold medal on the opening day of competition at the Aquatics Centre and is looming as almost unbeatable in the longest and most grueling race in the pool, after setting the world record at last year's world championships in Shanghai.

Churning through the water with seemingly effortless ease, Sun won his heat in a time of 14 minutes, 43.25 seconds, while Tunisia's Oussama Mellouli, the defending Olympic champion, was second fastest.

"I had some problems with my shoulder bugging me for the last three years but it seems to be working pretty well this morning," Mellouli said.

"The race went well, I'm feeling pretty confident for tomorrow."

Ranomi Kromowidjojo of the Netherlands set the fastest time in the semi-finals of the 50 freestyle to stay on course to complete the women's sprint double.

Back in the water after winning the 100 gold the previous night, Kromowidjojo exploded off the blocks and charged through her single lap in a time of 24.07.

"The semi-finals are about getting further and the final is what it is all about," she said.

(Editing by Greg Stutchbury)