The fourth-seeded American looked out of sorts in a 6-1 6-3 loss to world number 55 Donna Vekic from Croatia who caused problems with her big hitting.

Svitolina, from Ukraine, was beaten by German world number 57 Tatjana Maria.

Maria, one of a small number of mothers on the Tour, went through 6-7 (3-7) 6-4 6-1 after a late-night finish.

However, Caroline Wozniacki and Venus Williams were among the seeded players to go through.

Australian Open champion Wozniacki, the second seed, needed 59 minutes to beat American Varvara Lepchenko 6-0 6-3.

Five-time winner Williams, who was beaten by Garbine Muguruza in last year's final, made a slow start before beating Sweden's Johanna Larsson in three sets.

Larsson, who had never won in her seven previous Wimbledon visits, won the opening set on a tie-break.

But Williams, 38, grew in confidence to win 6-7 (3-7) 6-2 6-1.

Vekic's victory is her first over a top-five player and comes after first-round exits to Williams and Johanna Konta in the past two years.

"I always try to be aggressive," she told BBC Sport.

"I struggled with my serve in the wind but hopefully I can improve. I played all the tournaments on grass coming here and that has helped me."

The 2012 finalist Agnieszka Radwanska needed to save six match points before beating Romania's Elena-Gabriela Ruse.

The Pole, who has been struggling with back problems, went on to win 6-3 4-6 7-5.

American 16th seed Coco Vandeweghe was hampered by injury as she bowed out to Katerina Siniakova of the Czech Republic.

The 26-year-old needed treatment on her ankle during the match and was visibly limping as Siniakova eventually wrapped up a 6-7 (3-7) 6-3 8-6 win in three hours and one minute.