Grigor Dimitrov defeated Austria's Dominic Thiem 6-3, 5-7, 7-5 here in the opening match of the Pete Sampras Group at the ATP Finals.
World Number 6 Dimitrov, the first Bulgarian to qualify for the season-ending championship in the 48 years since its founding, battled for two hours and 21 minutes to beat fourth-seed Thiem on Monday, Efe news reported.
The 26-year-old Bulgarian broke his Austrian rival three times to win the match.
Thiem, ranked fourth in the world, won the Rio Open and was a finalist in Barcelona and Madrid.
He also reached the semifinals at Rome and the French Open.
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