LONDON (Reuters) - Michael Phelps took the morning off at the London Olympics on Friday but still managed to edge closer to winning yet another gold medal with a little help from his friends.

Despite swimming without their top lineup, the United States set the fastest qualifying time in the heats of the 4x100 meters medley relay on the penultimate day of swimming competition.

They will go into Saturday's final with all their big guns back and as the overwhelming favorites to win gold in what will be Phelps' last race at the Olympics before he retires.

"We're going to be in the stands waving flags," said Eric Shanteau, who swam the breaststroke leg in the heats. "None of us will be in the final relay so we just wanted to bring it home."

The U.S. have dominated the swimming events this week and are also heavily favored to win the women's medley relay but face a tough 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 Seebohm. "I am looking forward to the final."

Like the men, the Americans used their second-stringers for the heats, qualifying safely in fourth place, but will much faster in the final when the likes of Missy Franklin and Rebecca Soni are back in the lineup.

China's Sun Yang posted the fastest time in the heats of the 1500 freestyle to give himself the chance to become the first man in 32 years to complete the long-distance double.

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 heats 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 24.51 seconds to win her morning heat.

In an ominous warning to her rivals, Kromowidjojo said she had not slept as much as usual because of the excitement of her 100 win.

"I went to bed at 12 (midnight) and got up at 7(am)," she said.

"I was just thinking about gold, gold, gold. It's madness on my Twitter account."

The top 16 advanced to Friday night's semi-final which will decided the eight finalists for Saturday's last session of swimming in London.

Kromowidjojo's Dutch team mate Marlees Veldhuis was second, ahead of Britain's Francesca Halsall and Germany's Britta Steffen.