Chepkoech has won 15 of her last 17 steeplechases and is seeking a first world medal after a missed water jump wrecked her chances in 2017. There were no surprises in qualification for the women's pole vault, as all the top contenders cleared the required height with ease.
The reigning world and Olympic champion Katerina Stefanidi of Greece needed just a single vault to qualify at 4.60 meters, while 2012 Olympic champion Jennifer Suhr of the U.S. and her teammate Sandi Morris also qualified.
European indoor champion Anzhelika Sidorova from Russia also qualified. The pole vault offers good chances for a medal for Russia, which is competing in Doha as a team of "authorized neutral athletes" without a flag or anthem as punishment for past doping offenses.
Christian Coleman broke the 10-second barrier as he qualified fastest for the 100-meter semifinals at the world track championships on Friday.
The fastest man in the world this year, Coleman started strong and slowed the last 10 meters to finish in 9.98 seconds, weeks after the U.S. Anti-Doping Agency dropped his case for missed tests because of a technicality.
Earlier, defending champion Justin Gatlin beat Olympic silver medalist Andre de Grasse in their heat. Gatlin ran 10.06 seconds to beat the slow-starting de Grasse. At 37, Gatlin is three years older than any other man in the 100.
It's the first world championships without world record holder Usain Bolt since 2003.
"It feels a little different. Usually when he's around it's pandemonium," Gatlin said. "You see a lot of young guys. They're stepping up and filling that void."
NCAA champion Divine Oduduru of Nigeria was a surprise withdrawal from his heat. No reason was immediately reported.
The 2011 champion Yohan Blake won in 10.07 despite the slowest start in his heat. The fastest man besides Coleman was Akani Simbine of South Africa in 10.01, while Britain's Zharnel Hughes and Brazil's Paulo Andre Camilo de Oliveira also won their heats.
Sebastian Coe is closer to having a vote on the big Olympic decisions as the head of track and field's governing body.
IOC President Thomas Bach says he is "optimistic" clarifications over conflicts of interest are close to being gained that would allow Coe to become a member of the Olympic movement.
Bach says he first proposed Coe joined the roster of 105 active IOC members after the double Olympic champion was elected IAAF president in 2015.
Coe gave up his role as a special adviser to Nike in 2015 over accusations that he was vulnerable to conflicts of interest, but he remains chairman of sporting marketing company CSM.
Bach said after a meeting with Coe on Friday that "conflicts of interest must be different from the ones in IAAF. Here he is only responsible for one sport" but "as an IOC member you would have great responsibility for all the international federations and for the Olympic Games."
Bach added that "our ethics commission needed some clarification" and that after another meeting three weeks ago "I am optimistic these clarifications will be given by Seb Coe."
That would allow Coe to become an IOC member and vote on where the Olympics are held and which sports make the program.
The president of the IOC wants a "fresh look" at the Russian doping scandal amid allegations that critical lab data given to the World Anti-Doping Agency might have been manipulated.
Thomas Bach spoke Friday at a news conference before the start of the world track and field championships. He was sitting alongside Sebastian Coe, the president of track's international federation, which this week extended the suspension of the organization that oversees Russian track and field.
Bach said the IOC would respect WADA's rules. WADA has given Russia three weeks to explain the suspect data from the Moscow lab, which was being used to pursue cases against cheaters at the 2014 Sochi Olympics and other events.
Russia's anti-doping agency could be suspended again, though it is unclear how that would impact the country at next year's Olympics.
The women's 800 meters has started at the world track championships without its biggest name, Caster Semenya.
The South African won gold at the last two Olympics and at three world championships. She is barred from taking part this year because she refuses to follow new IAAF rules requiring her to medically reduce her natural testosterone level.
Ajee Wilson of the United States and Natoya Goule of Jamaica were among the gold medal contenders who won their heats, while 2013 champion Eunice Sum of Kenya barely scraped through as one of the fastest losers.
The fastest time across the six heats was just over 2 minutes by Winnie Nanyondo of Uganda, far off Semenya's personal best of 1:54.25.
Earlier this month, Semenya said she was looking forward to a "new journey" after signing with a soccer team. However, on Tuesday she posted a picture to Twitter of herself in a track bib reading "Doha," where the world championships are being held.
The track and field world championships have started in the Qatari capital Doha.
Long jumper Henry Smith of Australia was the first athlete to compete in men's qualification at the Khalifa International Stadium. Soon after, Taymir Burnet of the Netherlands won the first preliminary heat of the men's 100 meters.
The open-air stadium, which will be used for the 2022 World Cup, is air-conditioned. Temperatures outside hovered around 99 degrees (37 Celsius).
Later Friday, there is qualifying for events including the women's 800, before the women's marathon is held in at midnight on the streets of Doha.
The championships run through Oct. 6.
The midnight marathon is going on as scheduled at the steamy track and field world championships in Qatar.
Meet organizers notified the teams midday Friday that no changes in the schedule for the women's race were being made, and that they would reassess the situation after nightfall.
There had been speculation that the event might be postponed because of sweltering conditions. At 2:30 p.m. local time, it was 99 degrees (37 Celsius) in Doha with a "feels like" temperature of 122 (50 C). The forecast for the midnight race through downtown Doha is 89 (31.6 C).
There are 69 women on the start list, planning to race against each other and tackle conditions they rarely face. Meet organizers will have extra medical staff on the course looking for signs of heat exhaustion.
Inside the open-roofed stadium, where air conditioning is keeping the track about 20 degrees cooler than in the city where the marathon will be run, the meet will start Friday evening with qualifying for the men's 100 meters. Defending champion Justin Gatlin, American Christian Coleman and Canadian Andre De Grasse are among the medal contenders.