STUTTGART - World number four Petra Kvitova suffered a shock defeat in the WTA Stuttgart tournament on Thursday, beaten in straight sets by unseeded American Madison Brengle. Brengle, 25, ran out a 6-3, 7-6 (7/2) winner as Kvitova failed to make it past the second round in her first tournament since February. Brengle broke the 2014 Wimbledon champion's serve seven times to clinch victory in just over 90 minutes. Kvitova, who took a six-week break to recover from exhaustion, resumed training three weeks ago and won both her Fed Cup semi-final singles rubbers as the Czechs beat France 3-1 last weekend. The unseeded American, who reached the last 16 at the Australian Open in January, will face either France's Caroline Garcia or Carina Witthoeft of Germany in Friday's quarter-finals.