The Bulgarian, who had a bye into the second round, was put under pressure in both sets but needed just one match point to seal his tie-breaker victory.
Spain's Fernando Verdasco beat Jarko Nieminen of Finland 6-3, 6-4 to set up a quarterfinal match against Bernard Tomic.
The Australian beat third-seeded Kevin Anderson of South Africa 7-6 (4), 6-4.

In first-round matches, Jack Sock of United States defeated Andrey Golubev of Kazakhstan in three sets and France's Pierre-Hugues Herbert beat Pablo Carreno Busta of Spain.
German qualifier Dustin Brown beat Christian Lindell of Sweden 6-4, 7-6 (3) in the late match to reach the second round.

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