Michael Phelps shrugged off the disappointment of the United States' shock relay defeat by South Africa to easily qualify for the semi-finals of his favourite event at the Athens Olympics on Monday.

Phelps cruised to victory in the last heat of the men's 200 metres butterfly to safely advance to the semi-finals as the equal fastest qualifier in his first appearance in the pool since Sunday's 4x100 freestyle relay loss.

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His heat time of one minute 57.36 seconds was almost three and a half seconds outside the world record he set at last year's world championships but still enough for him to top the qualifiers along with Japan's Takashi Yamamoto, who set an identical time in a previous heat.

"It was a little hard after last night, it took a lot out of me emotionally because we really wanted that relay," Phelps said.

"But I'm not disappointed for me, I'm disappointed for the team."

Poland's Pawel Korzeniowski was third quickest, just 0.09 behind Phelps and Yamamoto, with America's defending champion Tom Malchow fourth.

STRONG FAVOURITE

Phelps already has one gold medal after winning the 400 individual medley on the opening night and is a strong favourite to collect another in his pet event.

Phelps finished fifth in the 200 fly at Sydney fours years ago when he was just 15 but broke the world record six months later and then won his first world title in 2001.

"I wanted a strong one this morning, I wanted to get into the stroke," Phelps said.

American Dana Vollmer overcame her nerves to top the qualifying for the women's 200 freestyle with a time of 1:59.49, finishing ahead of a stellar field.

Costa Rica's Claudio Poll, who won the gold medal in Atlanta eight years ago and has recently returned to swimming after serving a ban for a doping offence, was third, just 0.01 behind with Poland's Paulina Barzycka and Italy's Federica Pellegrini finished fourth. World record holder, Germany's Franziska van Almsick, was ninth.

"I normally don't get nervous but I was pretty nervous today. I told myself to calm down and it would be okay," Vollmer said.

"It felt awesome, I kept it smooth and I definitely feel I can go faster tonight."

Zimbabwe's Kirsty Coventry pipped Yana Klochkova as the fastest qualifier in the women's 200 individual medley after winning her heat in 2:13.33.

Klochkova won the medley double in Sydney and is on target to do it again after retaining her 400 title on Saturday.