Reigning world champion Jessica Ennis found herself trailing American rival Hyleas Fountain after the opening two events of the heptathlon at the world championships on Monday. Fountain, the Olympic silver medallist denied the chance to compete at the 2009 Berlin worlds because she failed to finish at US trials, ended the morning session on 2,228 points thanks to two season bests. The 30-year-old American timed 12.93sec over the 100m hurdles and managed a best of 1.89m in the high jump. Ennis recorded 12.94sec and 1.86m for a total of 2,187pts, with Russian Tatyana Chernova in third on 2,093. The heptathletes compete in the shot put and 200m in the evening session, and on Tuesday round off their efforts with competition in the long jump, javelin and 800m. All the favourites made it through the heats of the men's 400m hurdles, Welshman Dai Greene setting the fastest qualifying time of 48.52sec. South African duo Cornel Fredericks, who matched Greene's time, and L.J. van Zyl also looked in good form and will be joined in the semi-finals by 2004 Olympic champion Felix Sanchez of the Dominican Republic, a two-time former world champion. A strong US quartet including defending champion Kerron Clement, two-time Olympic gold medallist Angelo Taylor and 2005 world champion Bershawn Jackson also made it through. But it was team-mate Jeshua Anderson who set the lead US time of 48.81sec. Clement was left wobbling as he came across the line, but was confident he could offer more. "I hit the last hurdle but I will be fine for the semi-finals," he said. "This year I've had to fight several injuries and other problems but I will be back." Jackon was left lamenting the searing trackside conditions. "Today's run was extremely hard because of the heat and humidity. But the most important thing is that I made it through," Jackson said. Team-mate Taylor, the 32-year-old who won golds at the 2000 and 2008 Olympics, added: "I checked my fitness today. But we can't cruise in rounds to save energy because you have to run hard to keep the rhythm." The morning after Usain Bolt, the world's fastest man, was sensationally disqualified from the men's 100m final after a dreadful false start, athletics chiefs confirmed there were no immediate plans to discuss a rule change. "Rules are rules and those are the rules at the moment," an IAAF spokesman told AFP. "There has been no decision in terms of it being brought up at the next council meeting (at the end of the championships) that I know of." In a statement issued after Sunday's dramatic events, the IAAF said it was "disappointed" that Bolt had false-started but that a sport's credibility depended on its rules and they had to be applied consistently and fairly. The IAAF spokesman reiterated Monday that the Bolt disqualification had showed the essential fairness of the rules as not even athletics' biggest star and the globe's most marketable sportsman had been spared. "Bolt is still running in the 200m and the 4x100m. We need to give it a little bit of time. All of these reactions are very raw," he said. "We need to give a little bit of time to reflect." Evening action will see the three fastest sprint hurdlers in history -- Dayron Robles, David Oliver and Liu Xiang -- looking to book their berths in the 110m hurdles final later in the session. Allyson Felix will look to seal the first part of a 200m-400m double when she goes in the 400m final on a night that also sees finals in the women's 100m, men's pole vault, men's hammer and women's shot put.