Botvinnik held the world title, with two brief interruptions, for the next fifteen years, during which he played seven world championship matches. In 1951, he drew with David Bronstein over 24 games in Moscow, +5−5=14, keeping the world title, but it was a struggle for Botvinnik, who won the second-last game and drew the last in order to tie the match. In 1954, he drew with Vasily Smyslov over 24 games in Moscow, +7−7=10, again retaining the title. In 1957, he lost to Smyslov by 9½–12½ in Moscow, but the rules then in force allowed him a rematch without having to go through the Candidates' Tournament, and in 1958 he won the rematch in Moscow; Smyslov said his health was poor during the return match. In 1960, Botvinnik was convincingly beaten 8½–12½ at Moscow by Tal, now 23 years old, but again exercised his right to a rematch in 1961, and won by 13–8 in Moscow. Commentators agreed that Tal's play was weaker in the rematch, probably due to his health, but also that Botvinnik's play was better than in the 1960 match, largely due to thorough preparation. Botvinnik changed his style in the rematch, avoiding the tactical complications in which Tal excelled and aiming for closed positions and endgames, where Tal's technique was not outstanding. Finally, in 1963, he lost the title to Tigran Petrosian, by 9½–12½ in Moscow. FIDE had by then altered the rules, and he was not allowed a rematch. The rematch rule had been nicknamed the "Botvinnik rule" because he twice benefited from it.