Lieke Klaver

Since 2018, Klaver has also competed in the 400 metres, gradually shifting her focus to that distance, but without losing sight of the shorter event. In 2019, she won two medals for the 400 m at the Dutch Championships (indoors and outdoors). It earned her a place on the national 4 × 400 m relay team, which finished seventh at the Doha World Championships in Qatar that year.

In 2020, she claimed her first Dutch 400 m title (indoors), staying ahead of training buddy and rival Femke Bol. During this pandemic season, Klaver won also her first Diamond League race, a victory in the 400 m at the Golden Gala in Rome, Italy with a time of 50.98 s, becoming just the second Dutch woman to break the 51-second barrier.

Klaver represented Netherlands at the 2020 Summer Olympics in Tokyo, competing in the women's 400 m and both the women's and mixed 4 × 400 m relays. In the mixed 4 × 400 m relay, she competed in both the heats and final, in which the Dutch team finished fourth. In the women's 400 m, she was eliminated in the semi-finals. In the women's 4 × 400 m relay, she competed in the heats and final, helping the Dutch team finish in sixth place.

In February 2022, at the Dutch Indoor Championships, she set a new personal best for the indoor 400 m with a time of 51.20 s. The following month, at the World Indoor Championships, she finished sixth in the women's 400 m. In the women's 4 × 400 m relay, she ran the first leg in the heats and final, helping the Netherlands win the silver medal. During the summer season, Klaver greatly lowered her outdoor pre-2022 best (50.98 s) down to 50.18 seconds in the semi-finals at the World Championships in Eugene, Oregon, where she finished fourth in the final. She also competed in the mixed and women's 4 × 400 m relays. In the mixed relay, she ran the second leg in the heats and final, helping the Netherlands win the silver medal. In the women's relay, her team was disqualified in the heats.

In February 2023, in the 400 m race at the Dutch Indoors in which Femke Bol set a world record of 49.26 s, Klaver massively lowered her lifetime indoor best by nearly 0.7 s down to 50.34 s, putting her joint 13th on the world all-time list. Only two-time Olympic champion, Shaunae Miller-Uibo of the Bahamas, and Bol had run faster indoors since 2007. The following month, Klaver won her first individual major medal, a silver for the 400 m at the European Indoor Championships held in Istanbul.

Klaver's success carried over into the outdoor season. Throughout the season, she lowered her personal bests over 100 m, 200 m, and 400 m. Her biggest performance of the year came on 16 July at the Silesian Diamond League, where she lowered her 400 m personal best to 49.81 s, breaking the 50-second barrier for the first time in her career.

Later that year, Klaver had a busy schedule at the World Championship in Budapest. Aside from running the individual 400 metres event, she was also part of the Dutch relay squads in the mixed 4 × 400 metres, women's 4 × 400 metres, and women's 4 × 100 metres. On 19 August, during the 4 × 400 mixed relay final, Klaver's teammate Femke Bol fell within metres of the finish line, while vying with Alexis Holmes of the USA for first place. Landing face first, Bol lost the relay baton over the finish line, making it impossible for the Netherlands team to legitimately finish the race and resulting in a DNF.

After the disappointment, Klaver continued her championships in the individual 400 metres. After winning her semi-final in 49.87 s, she looked to be a potential medal contender. In the final however, she changed her race plan, starting much faster over the first 200 metres than usual. In the final 100 metres, she fell back to the middle of the field, resulting in a sixth place finish.

Later that week, Klaver was brought in for the women's 4 × 100 metres final. The Dutch team lost the baton on the last handover, resulting in another DNF.

Klaver ended her championships in the women's 4 × 400 metres final. During her second leg, she brought the Dutch team back from the middle of the pack to the top three, along with Great Britain and Jamaica. On the fourth leg, Klaver's teammate Bol anchored the Dutch team to the world title, passing the Brits and Jamaicans in the last metres. Individually, Klaver ran a split time of 48.78 seconds, which was the second fastest split of the race after Bol's 48.75 seconds.

On 17 September, Klaver was third in the 2023 Diamond League final of the 400 metres with a time of 50.47 seconds, after champion Marileidy Paulino and Natalia Kaczmarek.

Klaver won the 400 m short track races at the 2024 Czech Indoor Gala in Ostrava, Czech Republic in 50.54 s and at the 2024 Copernicus Cup in Torún, Poland in 50.57 s, and she finished second at the 2024 Meeting Hauts-de-France Pas-de-Calais in Liévin, France in 50.50 s. This made her the overall winner in the 400 m of the 2024 World Athletics Indoor Tour.

Klaver won a silver medal at the 2024 Dutch Indoor Championships in 50.10 s, after Femke Bol who won gold.

Klaver won a silver medal in the 400 metres short track at the 2024 World Indoor Championships in Glasgow, finishing in 50.16 s almost a second after Femke Bol. She continued to win a gold medal in the 4 × 400 metres short track relay with the Dutch women's team in a new Dutch record time of 3:25.07 min.

She won a gold medal at the 2024 Summer Olympics in the mixed 4 × 400 metres relay.