She explained the retirement on Twitter. Just a tweaked hamstring, so that's good.
I think last year she lost to Riske in first round of Nottingham, so slightly better result this time.
Originally Posted by @tamira1990
Tweaked my hamstring in the second set and didn't want to risk it at such an important time of year for me. Was feeling great out there though and so happy to feel that I've turned a corner. Loving being back on grass, Birmingham next!