Wimbledon champion Petra Kvitova and teammate Tomas Berdych won their singles matches on Monday to give Czech Republic a 2-0 win over Bulgaria at the Hopman Cup.
Kvitova won the last five games of her match to beat Tsvetana Pironkova 6-4, 6-2 before seventh-ranked Berdych defeated Grigor Dimitrov 6-4, 6-7 (8), 6-3.
The countries played a later mixed doubles match to determine the final scoreline, which could affect placings in Group A and decide which teams advance to Saturday's final.
On Monday night, the United States, represented by Bethanie Mattek-Sands and Mardy Fish, takes on a Denmark team comprising women's No. 1 Caroline Wozniacki and Frederik Nielsen.
No. 2-ranked Kvitova won five other tournaments in 2011 to add to her Wimbledon championship, then helped Czech Republic to the Fed Cup title. She was also named the WTA Tour player of the year.
"Of course the first match was tough and I won it," she said. "The last season was longer than normal and I had some rest. Hopefully I am ready to play matches - we will see how it goes."