Question: Frew McMillan and Billy Martin, have which previous occupation in common?

Context paragraphs:
Frew McMillan: Frew Donald McMillan (born 20 May 1942) is a former professional male tennis player from South Africa who won five major doubles championships including three Wimbledons with Bob Hewitt.  Altogether, he won 63 doubles titles, surpassed only by the Bryan brothers, Todd Woodbridge, John McEnroe and Tom Okker.  He was also ranked No.1 in Doubles on the ATP Computer for a significant period from 1977 to 1979 when he was aged 37.

1969 British Hard Court Championships: The 1969 British Hard Court Championships was a combined men's and women's tennis tournament played on outdoor clay courts at The West Hants Club in Bournemouth in England.  It was the 40th edition of the tournament and the second edition in the Open Era of tennis.  The tournament was held from 28 April through 3 May 1969.  John Newcombe, seeded first, and Margaret Court won the first open singles titles while the men's team of Bob Hewitt and Frew McMillan and the women's team of Margaret Court and Judy Tegart won the doubles titles.  The poor state of the courts led to a protest by a number of players.  The tournament made a financial loss due to higher expenses compared to the previous edition and poor weather during the final two days.

1977 Fischer-Grand Prix – Doubles: Bob Hewitt and Frew McMillan were the defending champions and won in the final 6–4, 6–3 against Wojciech Fibak and Jan Kodeš.

1979 U.S. Pro Indoor – Doubles: Bob Hewitt and Frew McMillan were the defending champions, but Hewitt did not participate this year.  McMillan partnered Bob Carmichael, losing in the semifinals.

1976 Fischer-Grand Prix – Doubles: Bob Hewitt and Frew McMillan won in the final 6–4, 4–0 after Brian Gottfried and Raúl Ramírez were forced to retire.

1974 Munich WCT: The 1974 Munich WCT was a men's tennis tournament played on indoor carpet courts in Munich, West Germany.  The tournament was part of Red Group of the 1974 World Championship Tennis circuit.  It was the second edition of the event and was held from 1 April through 7 April 1974.  Frew McMillan won the singles title.

1978 Queen's Club Championships – Doubles: Anand Amritraj and Vijay Amritraj were the defending champions but lost in the second round to Bob Hewitt and Frew McMillan.

1979 Stella Artois Championships – Doubles: Bob Hewitt and Frew McMillan were the defending champions but only McMillan competed that year with Colin Dibley.

Billy Martin (tennis): Billy Martin (born December 25, 1956, in Evanston, Illinois, United States), is a right-handed former professional tennis player from the United States.  During his career he won 1 singles titles and 3 doubles titles.  He achieved a career-high singles ranking of World No. 32 in 1975.  Martin currently serves as the head coach for the UCLA Bruins men's tennis team, a position he has held for the past 24 years since 1994.  Martin, who played at UCLA, has a 14 straight top 5 NCAA team finishes and a 9 consecutive 20-win seasons.  He was named ITA (Intercollegiate Tennis Association) division 1 National Coach of the Year and is a member of ITA Hall of Fame.

1980 Fischer-Grand Prix – Doubles: Bob Hewitt and Frew McMillan were the defending champions but lost in the semifinals to Bob Lutz and Stan Smith.

Supporting paragraphs: Frew McMillan Billy Martin (tennis)
Answer: professional tennis player