<P> Since 1973, 26 men have been ranked No. 1 by the ATP, of which 17 have been year - end No. 1 . The current world number one is Rafael Nadal . </P> <P> Since the introduction of the ATP rankings the method used to calculate a player's ranking points has changed several times . As of 2011, the rankings are calculated by totaling the points a player wins in his best eighteen tournaments, subject to certain restrictions . For top players the counting tournaments are the four Grand Slam tournaments, the eight mandatory ATP World Tour Masters 1000 tournaments, the player's best four eligible ATP World Tour 500 series tournaments (the non-mandatory ATP Masters 1000 event in Monte Carlo may be substituted for one of these), and his best two results from ATP World Tour 250 series . Lower - ranked players who are not eligible for some or all of the top tournaments may include additional ATP 500 and ATP 250 events, and also ATP Challenger Series, and Futures Series tournaments . The ranking points of players who qualify for the year - end ATP World Tour Finals also include any points gained at that tournament, increasing their counting tournament total to 19 . </P> <P> Roger Federer holds the records for both the most total weeks at No. 1 (302) and most consecutive weeks at No. 1 (237). Pete Sampras holds the record for the most year - end No. 1 (six, all consecutive). Patrick Rafter spent the fewest time at No. 1 (one week). </P> <P> Lleyton Hewitt is both the youngest (20 years, 268 days) world No. 1 and youngest year - end No. 1, while Rafael Nadal is the oldest (31 years, 211 days) year - end No. 1 . Andre Agassi is the oldest No. 1 (33 years, 131 days). </P>

Who is ranked first in men's tennis