The biggest showdown in years is on, despite a shocking stumble from Usain Bolt.

The Olympic champion and Justin Gatlin are in the 100-meter final at the world championships after both won their semifinal heats Sunday in the most contrasting of ways.

Bolt, always the hesitant starter, stumbled out of the blocks and had to chase hard throughout his heat to qualify in 9.96 seconds.

The confident swagger ahead of the race immediately changed into a disgusted shaking of the head after the early error almost cost him the final.

"I think I hit my foot too hard," Bolt said. "I just stumbled. I really don't know what happened."

The Jamaican great had as many as seven racers to catch before winning the heat. Anxious, he spent the last 10 meters looking to his left to make sure he would make the final.

As much as Bolt's race was chaotic, Gatlin's heat was sheer perfection — fast out of the blocks, great acceleration and a lightning fast pumping stride before he started coasting. And he still finished in 9.77 seconds, a time Bolt has not come close to this year.

"Easy. Breezy," Gatlin said.

It more than showed the challenge Bolt faces in Sunday's final.

In the third semifinal heat, Tyson Gay qualified in the same time as Bolt, even though he didn't stumble. Asafa Powell also joined them in the final.

The 100 final takes center stage on a night with three other gold medals at stake.

Three years after the 2012 London Games, and now a mother, Jessica Ennis-Hill remains the heptathlon standout she once was. With two more solid performances in the long jump and the javelin on Sunday, the Olympic champion — then known as Jessica Ennis — closed in on her second world title, six years after her first.

With only the 800-meter race to come later Sunday, Ennis-Hill has 5,706 points, 86 points more than Nadine Broersen of the Netherlands. The top performer of the year, Brianne Theisen-Eaton of Canada, is third with 5,612 points.

With all three competitors pretty even in the 800, the biggest rivalry at this year's world championships could well be for silver.

Through the competition, the performances of Ennis-Hill have mostly been off the marks she produced in London, but she is still the one to beat. And she got some unfortunate help from 22-year-old teammate Katarina Johnson-Thompson, who looked like Ennis-Hill's biggest challenger until she fouled out on all three of her long jump attempts.

Despite two fouls to start, Johnson-Thompson went all out on her last attempt and appeared to step over the line by a tiny margin on a huge jump. Despite pleading with the judges, the red flag went up.

The Chinese fans wanted their red flag, the national one, up on the highest pole but the host nation again fell short of a gold medal when Miguel Angel Lopez of Spain held off a strong challenge from Wang Zhen to win the 20-kilometer walk.

Even though Wang had been undefeated for a year, he entered the Bird's Nest when Lopez was already moving toward the finish line celebrating a gold medal to add to his European title from last year. Benjamin Thorne of Canada was third.

"I felt some pressure as this competition takes place in China," Wang said.

On Saturday's opening day, Christina Schwanitz of Germany relegated Gong Lijiao to a silver medal in the women's shot put.