Association of Tennis Professionals

The Association of Tennis Professionals (ATP) was formed in 1972 to protect the interests of male professional Tennis players. (Female players formed the Women's Tennis Association the year after.) In 1990, the association became the organizer of the principal worldwide tennis tour, thereafter known as the ATP Tour.

The organization releases a weekly ranking of the players's performance in the tour. This is a list of players who have achieved the number one position in singles since the ranking's inception in 1973:

#	Player	Country	Date Reached	Age	Weeks
1	Ilie Nastase	Romania	23 August 1973	27y 1m	40
2	John Newcombe	Australia	3 June 1974	30y 11d	8
3	Jimmy Connors	United States	29 July 1974	21y 11m	268
4	Bjorn Borg	Sweden	23 August 1977	21y 2m	109
5	John McEnroe	United States	3 March 1980	21y 15d	170
6	Ivan Lendl	Czechoslovakia	28 February 1983	22y 11m	270
7	Mats Wilander	Sweden	12 September 1988	24y 1m	20
8	Stefan Edberg	Sweden	13 August 1990	24y 9m	72
9	Boris Becker	Germany	28 January 1991	23y 2m	12
10	Jim Courier	United States	10 February 1992	21y 5m	58
11	Pete Sampras	United States	12 April 1993	21y 8m	286
12	Andre Agassi	United States	10 April 1995	24y 11m	88
13	Thomas Muster	Austria	12 February 1996	28y 4m	6
14	Marcelo Ríos	Chile	30 March 1998	22y 3m	6
15	Carlos Moyà	Spain	15 March 1999	22y 6m	2
16	Yevgeny Kafelnikov	Russia	3 May 1999	25y 2m	6
17	Patrick Rafter	Australia	26 July 1999	26y 8m	1
18	Marat Safin	Russia	20 November 2000	20y 10m	9
19	Gustavo Kuerten	Brazil	4 December 2000	24y 2m	43
20	Lleyton Hewitt	Australia	19 November 2001	20y 9m	76
21	Juan Carlos Ferrero	Spain	8 September 2003	23y 7m	7
22	Andy Roddick	United States	3 November 2003	21y 2m	13
23	Roger Federer	Switzerland	2 February 2004	22y 6m	present