Rafael Nadal regained the number one spot ahead of the French Open in the world rankings on Monday after winning his eighth Rome Masters title.
The victory saw Nadal overtake Roger Federer, who is not playing the claycourt season this year.
Former number one Novak Djokovic continued his fall, slipping four places to 22nd.
ATP rankings at May 21:
Rafael Nadal (ESP) 8,770 pts (+1)
Roger Federer (SUI) 8,670 pts (-1)
Alexander Zverev (GER) 5,615
Marin Cilic (CRO) 4,950 (+1)
Grigor Dimitrov (BUL) 4,870 (-1)
Juan Martin Del Potro (ARG) 4,450
Kevin Anderson (RSA) 3,635
Dominic Thiem (AUT) 3,195
David Goffin (BEL) 3,020 (+1)
John Isner (USA) 2,955 (-1)
Pablo Carreno (ESP) 2,415
Diego Schwartzman (ARG) 2,165 (+3)
Sam Querrey (USA) 2,140 (-1)
Roberto Bautista (ESP) 2,120 (-1)
Jack Sock (USA) 2,110 (-1)
Lucas Pouille (FRA) 2,030
Kyle Edmund (ENG) 1,950 (+2)
Tomas Berdych (CZE) 1,900 (-1)
Fabio Fognini (ITA) 1,895 (+2)
Hyeon Chung (KOR) 1,775
Selected:
Novak Djokovic (SRB) 1,665 (-4)
Richard Gasquet (FRA) 1,395 (-2)
Jo-Wilfried Tsonga (FRA) 1,245