Allyson Felix struggled through her first race in 13 months on Thursday but drew positives from her performance after scraping through her opening heat in the 400m at the US Championships.

Felix, 33, the only woman ever to win six track and field Olympic gold medals, is on the comeback trail after taking a year out following pregnancy and the birth of her daughter in December.

On Thursday at Drake Stadium, Felix found the going tough, finishing fourth in her heat in a time of 52.20sec, way outside her personal best of 49.26 set in 2015.

That left the veteran former 200m and 400m world champion sweating on the outcome of other races to confirm her spot in the next round.

Felix later qualified via one of the four spots available to finishers outside the top three.

“It felt rusty, kind of to be expected,” Felix said afterwards. “I haven’t raced in a long time. It’s going to take some time to get back into it.”

Felix has set her sights on competing at a fifth Olympic games in Tokyo next year and said Thursday’s display had fuelled her conviction that she remains capable of competing at the highest level.

“Tonight I’m just grateful to be back racing,” she said. “Even though it wasn’t a great result for me, it’s a starting point.

“My biggest goal is next year. And I know that I’m capable. Now I have time on my side and I can get where I need to go.”

Felix is currently racing without a sponsor after failing to renew a deal with Nike, who she recently criticised for their policy towards female athletes who become pregnant, admitted that she was still adjusting to the twin demands of training for her comeback and parenting.

“Things that were pretty easy for me are now pretty challenging,” Felix said of her training.

“So just getting past that. And being a mom, I’m a regular mom, at the hotel I’m cleaning bottles and changing diapers, and getting ready for racing.

“It’s hard. Very hard. But I’m kind of getting the hang of it more. But I have a great support network right now.”

Before taking to the track Thursday, Felix shared a photo of herself and her daughter Camryn in the newborn intensive care unit. Her daughter was born on November 28 at only 32 weeks after Felix had to have an emergency C-section.

“Staying in the NICU all day & night watching my baby girl fight. I can still hear the beeping and alarms of the machines. The uncertainty. The fear,” she wrote.

Speaking to reporters on Thursday, Felix said regaining her physical conditioning had been the hardest part of her comeback.

“Physically. My body went through a lot,” she said. “I’m not even where I want to be strength wise. I’d say that’s the hardest part.

“It’s very humbling. Extremely humbling. But also I’m just happy to be running. Because I wasn’t sure that I was going to be the case. But I’ve got a ways to go and I know that I can still run.”