<human>: Rodney George Laver  (born 9 August 1938) is an Australian former tennis player widely regarded as one of the greatest in the history of the sport. He was the No. 1 ranked professional from 1964 to 1970, spanning four years before and three years after the start of the Open Era in 1968. He also was the No. 1 ranked amateur in 1961-62. Laver's 200 singles titles are the most in tennis history.

Laver had a long-running, friendly rivalry with Ken Rosewall between 1963, when he started out as a pro, and 1976, when both were semi-retired from the main tour. Including tournaments and one-night stands, they played over 130 matches, all of them as professionals, with some results from the barnstorming pro tours lost or badly recorded. Overall a match score of 79-63 in favour of Laver can be documented.  Against the older Pancho Gonzales, whom he played 1964 to 1970 on the pro tour, Laver had a lead of 35-19 or 38-21, depending on the source.  Laver had another, even longer rivalry with his fellow Queenslander Roy Emerson. They met first on the senior amateur tour in 1958 and dominated the amateur circuit until 1962, before Laver turned pro. When open tennis arrived in 1968, Emerson joined the pro tour, and had many new battles with Laver. Overall the score is 49-18 in favour of Laver, with 7-2 in major Grand Slam tournaments.  Laver had also many battles with Lew Hoad in his first years on the pro circuit 1963-1966. Although he lost the first 8 matches in January 1963, Laver later in the year began to turn around their rivalry, and until 1966, he had built a 38-21 lead. Against Arthur Ashe, Laver had a head-to-head lead of 21-3, winning all of the first 18 matches. Ashe's first win came in 1974, when Laver was 35. Another younger rival in the Open Era was John Newcombe, whom Laver led 16-5 in their head-to-head score. CANNOTANSWER

Using a quote from the above article, answer the following question: Who was his biggest rival?
<bot>: Ken Rosewall