LONDON (Reuters) - Michael Phelps, relaxed and unflustered by his slow start to the London Olympics, cruised through the heats of his favorite event on Monday to edge closer to two more amazing milestones.

The American swam well within himself, conserving his energy for his best races which are still to come, to qualify fifth fastest overall in the preliminaries of the 200 meters butterfly.

Austria's Dinko Jukic was the fastest overall, stopping the clock at one minute 54.79 seconds, just ahead of America's Tyler Clary, but all the top swimmers just did enough to safely get through to the next round.

The fastest 16 advanced to Monday night's semi-finals from which the top eight go through to Tuesday's final.

"I'm pretty happy with that swim," Phelps said. "That's all I needed it to be."

Phelps won the event at Athens in 2004 and again in Beijing four years later. If he wins again on Tuesday, he would become the first male swimmer to win gold in the same individual event at three successive Olympics.

He will also equal Soviet gymnast Larisa Latynina's record tally of 18 Olympic medals and could claim the outright record the same night in the 4x200 freestyle relay event.

Chinese sensation Ye Shiwen set herself up for a golden double when she posted the fastest qualifying time in the women's 200 individual medley then dismissed suggestions of foul play.

The 16-year-old shattered the world record to win the 400 medley on Saturday, but her eye-popping performance, when she swam her final lap almost as fast as Ryan Lochte did in the men's race, aroused suspicions.

"There's absolutely no problem with the doping,' she told reporters, speaking through a translator. "The Chinese team has always had a firm policy about anti-doping."

Despite easing off on the final freestyle leg, Ye still finished well clear of her main rivals to emerge as the nearly unsinkable favorite to snatch the gold.

She was more than a second and a half ahead of her nearest rival, Zimbabwe's Kirsty Coventry, and more than three seconds clear of Australia's Stephanie Rice, the Beijing Olympic champion and Ariana Kukors, the current world record holder.

Federica Pellegrini rebounded from the disappointment of missing a medal in the 400 freestyle final by setting the fastest qualifying time in the 200 freestyle heats.

The Italian finished a disappointing fifth in the 400 final on Sunday after going into the event as the reigning world champion and world record holder but made a better start to her defense of the 200 gold medal she won in Beijing four years ago.

Racing in the lane next to American teenager Missy Franklin, Pellegrini stopped the clock at 1:57.16, ahead of Allison Schmitt, who won the silver medal in the 400, and Franklin.

"My time was OK, now I will concentrate on the semis and take it just one step at a time," Pellegrini said.

Camille Muffat of France, who won the 400 gold medal, was 12th overall. The top 16 swimmers qualified for the semi-finals later on Monday, with the fastest eight from the semi-finals advancing to Tuesday's final.

"I couldn't sleep (last night)," said Muffat. "I would open the drawer, take out my medal and say 'oh yeah, it's true, I won'.

"One is what I wanted, two, I never thought possible. A medal of any color would be something."