Roger Federer drops out of the top three, which had been occupied by the Swiss veteran along with his rivals Rafael Nadal and Novak Djokovic constantly since May 2019.
Thiem has had something of a breakthrough 12 months, shrugging his clay court specialist tag to win titles on the hard courts of Beijing, Vienna and most notably Indian Wells, while also making the final of the ATP Finals in November.

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He has made a fine start to 2020, reaching the Australian Open final, where he lost to Djokovic, courtesy of fine performances against Nadal in the quarter-finals and Alexander Zverev in the last four.
He had held a 2-1 lead over Djokovic in the final after three sets but the Serb was ultimately able to stage a comeback and claim his eighth Melbourne title.
In his first tournament since the season's first Grand Slam, he reached the quarter-finals in Rio last week.
Federer drops down after losing his Dubai title, which marked his 100th tournament win in the 2019 season. The 38-year-old is set to be missing until the grass court season after undergoing knee surgery after the Australian Open.
Djokovic has extended his lead at the top of the rankings by winning his fifth Dubai title last week, defeating Stefanos Tsitsipas in the final.
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