https://www.atptour.com/en/rankings/singles
In the Dubai tournament, D. Thiem lost to Harris which was quite surprising, and in 2021 he has not yet shown an impressive performance.
For me this doesn't mean much to see Medvedev overtake Nadal in the rankings as Medvedev is still yet to win a major tournament. He has played in 6 more tournaments then Nadal so it was coming for him to overtake Nadal. Most likely Nadal will reach the French Open final and maybe even win it which will see him take back the number 2 ranking and put a decent sized gap between him and Medvedev.
Thiem seems to be down on form this year but hopefully the clay court season will see him find some form again like we have seen in the past. For some reason Thiem seems to have a good mid to end season but not so good at the start of the year.