The No.11-seeded Vinci was the first to fall, with Bulgarian qualifier Tsvetana Pironkova storming out to a 63 40 lead with points for 63 50 and, after a slight stumble, closing it out, 63 63. It was a little bit of a revenge mission for Pironkova - Vinci had prevailed in all three of the pair's previous meetings.
"I started really solid - my concentration got lost a little bit, but I'm happy the way I played overall," Pironkova said. "We played three or four times before and I never won, so this is great for me."
The No.14-seeded Stephens fell in the night session to Czech qualifier Petra Cetkovska. Things looked dead even at 5-all in the first set, and Stephens even looked like she was about to pounce in that game as she brought up two break points - but Cetkovska held and would eventually win eight of nine games to run away with it, 75 61. The big-hitting Czech reeled off the last seven points of the match.
"I'm very, very happy with the win today," Cetkovska said. "The most important thing was I stayed as aggressive as I could be, even though I was making a couple of errors. I just stuck to it today.
"Every single win is important, no matter who it's against. Against Sloane and the top players it's even bigger but of course every win will give me confidence going forward into the new season."
Cetkovska also had a little bit of revenge - Stephens had won their only two previous meetings.
Other winners on Day 1 were Venus Williams, Peng Shuai, Hsieh Su-Wei and Annika Beck. Williams put on a serving display en route to a 62 62 victory against Petra Martic (read more here). Peng and Hsieh both edged former Top 10 players - Peng battled back from a break down in the first set and 5-3 down in the second set to beat former World No.3 Nadia Petrova, 76(5) 75, while Hsieh twice rallied from a break down in the first set en route to a 75 63 win over former World No.10 Flavia Pennetta.
"I know Nadia was injured and off the tour for a while and is just coming back, and she came through qualies this week," Peng said. "But she's always a tough player. She has a big serve and hits a big ball. It was also a little bit windy today and not easy to play, so I just had to focus on my game and fight.
"In the second set I was down 3-0 but I kept fighting and I'm happy I made it through this one."