Sitak and American partner Nicholas Monroe battled for over two hours before Marcin Matkowski (Poland) and Max Mirnyi of Belarus closed out a 6-3 3-6 6-3 6-4 win.
Daniell and Brazil's Marcelo Demoliner worked even harder in their match against Neal and Ken Skupski.
It took five minutes shy of three hours, but the British brothers eventually triumphed 7-6(7-3) 5-7 7-6(9-7) 6-4.
Venus is also juggling a mixed doubles campaign with Barbora Krejcikova of the Czech Republic.
The pair are through to the third round after beating Mikhail Elgin (Russia) and Anastasia Rodionova (Australia) 4-6 6-3 6-4, and face a tough challenge against second seeds Bruno Soares of Brazil and Elena Vesnina (Russia).