Rome, July 28 (DPA) An astonishing eight swimmers managed to swim under the championship record in the heats of the men's 50m breaststroke, as the third day of swimming got underway Tuesday at the world championships here.

Also in action early Tuesday was American superstar Michael Phelps, who comfortably qualified for the semi-finals of the men's 200m butterfly.

In the women's 200m freestyle heats, Swedish teenage sensation Sarah Sjostrom, who has already broken two world records and has won one gold medal, failed to qualify for the semi-finals.

The 15-year-old, who is not considered a 200m freestyle specialist, finished her heat in a time of one minute 58.58, well behind American Dana Vollmer, who had the best time in 1:55.98.

Her compatriot Allison Schmitt had the second-fastest time, 0.77 seconds behind Vollmer.

In the men's 800m freestyle, it was Oussama Mellouli who lead the swimmers into the final. The Tunisian swam a 7:41.82, edging out Canadian Ryan Cochrane by nearly two seconds.

Italian Federico Colbertaldo was cheered as he won his heat in a time of 7:44.29 for the third-fastest time.

In the men's 200m butterfly Phelps was in complete control in his heat and although he did not qualify for the semi-finals with the fastest-time, he was just one place below Poland's Pawel Korzeniowski, who topped the qualifying with a time of 1:54.33.

Phelps was just two hundredths slower, while his compatriot Scott Tyler Clary was third-fastest.

Phelps said that he was happy with his swim. 'I just wanted to win my heat, that is all I wanted to do. One of the biggest things about that event is getting into the finals.