This after losing in the third round of the Masters Tournament in Indian Wells, California.
The 28-ranked Frenchman Gaël Monfils won 4-6, 6-3, 6-1.
It was the first time since 2009 that 35-year-old Monfils beat a world number one.
For the Frenchman, it was extra nice to beat a Russian opponent, as he is married to the Ukrainian tennis star Elina Svitolina.
After the match, he slapped himself on the chest and pointed to Svitolina who was sitting in the stands.
Next week, Serbia's Novak Djokovic regains first place in the rankings.