By Will Swanton NEW YORK (Reuters) – Petra Kvitova required blood tests for another mystery virus before being eliminated from the U.S. Open on Saturday by an American who travels the world with her childhood security blanket. Kvitova, the former Wimbledon champion, was on the verge of becoming world number one last year. But her decline in results continued with a lackluster 6-3 6-0 loss to world number 81 Alison Riske in the third round at Flushing Meadows. …