Michael Phelps shook off a rusty 150 meters to storm home in the final lap and qualify for the men's 200 meters butterfly semi-finals at the world swimming championships on Tuesday.

The Olympic champion, who is also in the final of the 200 freestyle later on Tuesday, clocked one minute 56.77 seconds to finish first in his heat and 11th overall.

Phelps said he was not concerned with the time as his job had been to qualify for the semi-finals, conserve his energy and concentrate on the 200 freestyle final, something that did not escape the notice of China's Wu Peng, who qualified 15th.

"Michael's got another competition tonight so he also saved energy," Wu told reporters.

The 26-year-old Phelps is swimming four individual events in Shanghai -- the 100 and 200 butterfly, 200 freestyle and 200 individual medley -- as well as three relays as he prepares for next year's London Olympics.

Phelps led off the U.S. 4x100 freestyle relay on Sunday, but the heavily fancied team were usurped by a young Australian quartet and the French and had to settle for bronze.

Italy's 400 freestyle world champion Federica Pellegrini cruised into the semi-finals of the 200 freestyle with the second fastest time, behind Allison Schmitt of the U.S.

"I'm very satisfied," Pellegrini said. "In Rome 2009, I was slow in the morning. I always want to be fast.

"I'll try my best in the semi-final and final of the race. But it's always difficult to finish the 200m race."

Norway's Alexander Dale Oen, who channeled the grief his country was experiencing after a mass shooting and bombing in Oslo that killed 76 people to win the men's 100 breaststroke on Monday, qualified fifth fastest for the 50 breaststroke semi-finals.

"I felt a little tired and a little slow after yesterday. The main thing now is just qualifying for the semi," he said.

"The next thing is to rest up a little bit and get the energy restored."

China's Sun Yang, the silver medalist in the men's 400 freestyle on Sunday, qualified fastest for Wednesday's 800 freestyle final.