In the Doubles competition of the 1988 Virginia Slims of California tennis tournament, Hana Mandlíková and Wendy Turnbull were the defending champions but they competed with different partners: Mandlíková with Jana Novotná and Turnbull with Zina Garrison.
Alexander Volkov was the defending champion, but lost to Daniel Vacek in the quarterfinal.
Carl-Uwe Steeb won in the final 7–6, 3–6, 7–6 against Daniel Vacek.
Cyril Suk and Daniel Vacek were the defending champions but they competed with different partners that year, Suk with Petr Korda and Vacek with Yevgeny Kafelnikov.
Jan Siemerink and Daniel Vacek were the defending champions, but did not play together this year.
 Siemerink partnered Martin Damm, losing in the quarterfinals.
 Vacek partnered Alexander Mronz, losing in the first round.
Daniel Vacek (born 1 April 1971) is a former tennis player from Czechoslovakia and the Czech Republic who turned professional in 1990.
 He reached the quarterfinals of the 1995 Paris Masters, the 1998 Canada Masters and the 1998 Cincinnati Masters, and achieved a career-high singles ranking of World No. 26 in January 1996.
Bettina Fulco (born 23 October 1968) is a retired Argentine professional women's tennis player.
 She reached her highest ranking of No. 23 on October 10, 1988.
 Fulco began playing tennis at age 10 at the University Club in her hometown of Mar Del Plata, having been inspired to start because of the increased interest in the sport in Argentina due to Guillermo Vilas' success.
 As a junior, Bettina was among the best in the world, reaching the finals of the Orange Bowl 18-and-under championships in 1986, and finishing second in the rankings in 1986.
 She turned professional in 1987.
 Like many South American players, Bettina Fulco was considered a clay court specialist, and reached the quarterfinals of the French Open in 1988.
 Bettina beat Martina Navratilova in Houston 1994 for her biggest career victory.
 She also achieved victories over Conchita Martínez, Arantxa Sánchez Vicario, Hana Mandlíková, Katerina Maleeva, Manuela Maleeva, Magdalena Maleeva, Claudia Kohde-Kilsch, Lori McNeil and Nathalie Tauziat.
 She retired from professional tennis in 1998.
The 1993 Volvo International was a tennis tournament played on outdoor hard courts at the Cullman-Heyman Tennis Center in New Haven, Connecticut, United States and was part of the Championship Series of the 1993 ATP Tour.
 The tournament ran from August 16 through August 23, 1993.
 Kelly Jones and Rick Leach were the defending champions but only Jones competed that year with Paul Annacone.
 Annacone and Jones lost in the semifinals to Cyril Suk and Daniel Vacek.
 Suk and Vacek won in the final 7–5, 6–4 against Steve DeVries and David Macpherson.
Hana Mandlíková and Wendy Turnbull were the defending champions but they competed with different partners that year, Mandlíková with Jana Novotná and Turnbull with Elizabeth Smylie.
Cyril Suk and Daniel Vacek were the defending champions but they competed with different partners that year, Suk with Hendrik Jan Davids and Vacek with Richey Reneberg.
Hana Mandlíková (born 19 February 1962) is a former professional tennis player from Czechoslovakia who later obtained Australian citizenship.
 During her career, she won four Grand Slam singles titles: the 1980 Australian Open, 1981 French Open, 1985 US Open, and the 1987 Australian Open.
 She was also the runner-up at four Grand Slam singles events, including the Wimbledon finals of 1981 and 1986, and won one Grand Slam women's doubles title, the 1989 US Open with Martina Navratilova.

Question: In which sport did the Czech stars Daniel Vacek and Hana Mandlíková gain professional status?
Answer: tennis