A second kiwi crew, Markus Somerville and Aaron Hume-Merry, are also lying within the top 10 with three single-digit results (9, 5, 9) from the first day of competition.
Currently standing in the top three places in the 49er are Croatia's Kostov and Cupac leading, Spain's Paz Blanco and Paz in 2nd while Poland's Przybtek and Kolodzinski are 3rd.
The NZL Sailing Team's 49erFX pair of Alexandra Maloney and Molly Meech are lying 20th.
They sailed four races on Monday placing 12th, 11th, 4th and 9th.
The regatta, which has attracted 61 boats in the men's 49er fleet and 44 in the women's 49erFX fleet, had another five days of racing scheduled before medals were decided.
Racing resumed yesterday with the women racing first.