In 2018, Hiltz signed with Adidas and trained with The Mission Athletics Club, now known as The Golden Coast Track Club based in California after running a 1500 m time ranked in the top 100 in the world.[13] In January of the same year they ran a then-personal best 2:05.16 in the 800 m. Later in the year Hiltz ran the Aetna Falmouth Mile placing 4th in 4:32.29. They followed up with a 5th-place finish at the Memphis Ed Murphey Mile with a time of 4:32.59. They also ran the Bay ShoreHoka One Long Island Mile placing 12th in 4:39.23. Hiltz ranked 92nd in the world in the 1500 m with a personal best of 4:09.14.
2019
Hiltz ran an indoor 3000 m (9:04.32) at the JDL Fast Track Invitational, won the indoor mile (4:31.42) at the Husky Classic, and at the 2019 USA Track & Field Indoor Championships placed 5th in the mile (4:32.40) and 15th in the 2 mile (9:55.50). Later that year, Hiltz won the 800 m (2:01.37) at the 2019 Bryan Clay Invitational, won the 1500 m (4:07.71) at the 2019 USATF Distance Classic, runner-up in the 1500 m (4:05.56) at 2019 Portland Track Festival, won the 800 m (2:02.93) at the 2019 Adrian Martinez Classic, won the 1500 m (4:05.97) at the 2019 Sunset Tour. They also placed 3rd in the 1500 m (4:03.55) at the 2019 USA Outdoor Track and Field Championships. Hiltz ranked 19th in the world in the 1500 m with a personal best of 4:01.52 in the semi-finals of the 2019 World Athletics Championships.[14]
2020
Hiltz ran the mile at the Millrose Games in 4:24.45, splitting a 1500 m time of 4:07.09 and placing 5th with an indoor personal best.
In the summer of 2022, no longer sponsored by Adidas, Hiltz announced that they had signed with Lululemon.[16] On September 11, Hiltz finished second in the 2022 the Fifth Avenue Mile in 4:17.4 behind winner Laura Muir.[17]
2023
Nikki Hiltz won their first outdoor USATF Outdoor National Championship title in the 1500 in 4:03.10, moving from 3rd to 1st in the last 300m.[18] On July 21, 2023, Hiltz finished 6th in the mile at the Monaco Diamond League in a time of 4:16.35, a personal best, North American Area Record[19] and an American Record.[20][21]
2024
In the first race of the 2024 season, Hiltz won the 1,000-meter run at the Mile City Mayhem Event at the UW Invitational in 2:34.09, setting a personal best and American Indoor Record.[22][23]
In an effort to test endurance, Hiltz decided to contest the 2 mile at the 2024 Millrose Games rather than their specialty event, the mile. Competing on February 11, they placed 4th in a time of 9:15.80 which improved upon their personal best set in 2019 by almost 40 seconds. The following week, Nikki contested the 1500 m at the US Indoor Championships in Albuquerque, successfully defending their US indoor title in a time of 4:08.35. Placing top two, alongside Emily Mackay, qualified Hiltz to compete at the 2024 World Athletics Indoor Championships in Glasgow.
In the qualifying round, on March 1, Hiltz posted the fastest time of the heats, running an indoor personal best of 4:04.34. In the final, Hiltz took second place behind Freweyni Hailu in a personal best of 4:02.32, earning a first global medal and becoming the first American to medal in the event since Regina Jacobs in 2003. Hiltz was joined on the podium by fellow American Emily Mackay who took bronze.
In the 2024 US Olympic Trials, Hiltz competed in the 1500m run and made it through the preliminaries and semifinals. In the final, Hiltz won in a time of 3:55.33, thereby making the Olympic Team, and also setting a new trials record.[24]
At the 2024 Paris Olympics, Hiltz finished 3rd in both their preliminary heat and semifinal heat, qualifying for the final. In the Women's 1500 meter final, Hiltz finished seventh with a time of 3:56.38.[5]
In 2020, Hiltz founded Pride 5k, an annual charity race that has raised over $172,000 for The Trevor Project, a non-profit organization focused on suicide prevention efforts among LGBTQ youth.[25][26][27] Thousands of people have supported the race.[28][29]
*Distances have varied as follows: Mile (1967–2002), except 1997 and 1999, 2007 and odd numbered years since 2011, 1500 meters 1997, 1999, (2003–6, 2008–2010) and even numbered years since 2010