In 1984, young Kasparov faced veteran Karpov for the World Chess Championship. Karpov started off with a 4-0 lead in a race to 6 wins, but after 48 games and 5 months, the score was 5 to 3, the rest being draws. Karpov was looking exhausted, and the entire Championship was called off by the FIDE president, despite objections from both players. The two met again in 1985 and Kasparov won the title. (source)

In September 2009, Kasparov and Karpov will meet in Valencia for a symbolic 3 day rematch. (source)