The highly rated combination of Soares and Peya - who reached the final of the 2013 US Open and have two Masters titles to their credit - were expected to prevail but didn't count on the skill and resilience of the Kiwi-Austrian pair.
After grabbing an early break in the first set, Venus and Marach served it out in 35 minutes.
Peya and Soares lifted the intensity in the second set but were unable to convert a couple of break point opportunities on the Marach serve.
The fifth seeds also held a set point at 6-5, 30-40, but the New Zealand No1 Venus saved it with a brilliant wide serve.
The tie-break had everything - aces, double faults, superb volleying and unbelievable rallying - and some of the best tennis of the match.
After edging to a 6-5 lead, Venus and Marach converted their first match point to the delight of the many Kiwis in the crowd.