Tennis: US get off to unexpected flier in Cup
Mattek-Sands wasted no time in applying pressure to French Open champion Schiavone, breaking to go 3-2 ahead in the opening set and maintaining her lead the rest of the way.
Schiavone looked under par on another humid day, falling a break behind at the start of the second set. She broke back, but it was merely delaying the inevitable as Mattek-Sands broke again to complete the victory.
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Hide AdMeanwhile, champion Yanina Wickmayer joined top seed Maria Sharapova in the quarter- finals of the Auckland Classic, while Svetlana Kuznetsova squandered a three-game lead in the final set and crashed out.
Sharapova, despite some stuttering, roared to a 6-3, 7-5 victory over Renata Voracova of the Czech Republic, while second seed Wickmayer had to battle to beat German qualifier Sabine Lisicki 6-2, 3-6, 6-2.
Third seed Kuznetsova was bundled out 2-6, 6-4, 6-4 by China's Peng Shuai after she blew what should have been a match-winning lead in the final set.
Sharapova followed Kuznetsova on to the newly-laid Plexicushion courts and after going 3-1 down in the first set she blasted the 27-year-old Voracova away, losing just one point in the final four games of the set.
The second set was just as erratic, with Voracova jumping out to a 5-3 lead before the US-based Russian stormed back to win the next four games and book a quarter-final against Hungary's Greta Arn.
Kuznetsova, however, will make an earlier than planned flight for next week's tournament in Sydney after she failed to capitalise on her opportunities against Peng.
She romped through the first set but tightened up in the second to let the Chinese player force the second-round match to a decider.
Kuznetsova raced to a 3-0 lead in the final set but again failed to hammer home the advantage and instead Peng ground the 25-year-old Russian down, sealing the victory in two hours 13 minutes.