The ACE Pressure Point of the Match: Stephens earned her only break point of the first set in the ninth game, and didn't let the opportunity pass her by. After an exchange of 20 shots, the American came into net after striking a low slice, then struck a high volley for the lead.
Kerber, seeded second, was making her first appearance in New Haven. She drew a first-round bye.
Vesnina, a 2009 finalist, won the first three games to take control of the first set. She broke serve for the sixth time in the final game of the match. Kerber hit a forehand just wide on match point, then threw her racket in disgust. The call was upheld after a challenge.
Later Tuesday, defending champion Petra Kvitova faces Annika Beck in a match postponed because of rain Monday night.