NEW YORK, Sept 5 (Reuters) - World number two Aryna Sabalenka overcame early mistakes to advance past Belgian Elise Mertens 6-4 6-1 on Sunday, reaching the U.S. Open quarter-final round for the first time. Up an early break, the Belarusian appeared to lose one of her most reliable weapons - her power serve - as she committed three double faults in the third game to help 15th-seeded Mertens level the score 2-2. She ended the set with eight double faults and 15 unforced errors but managed to convert a break point in the final game on Louis Armstrong Stadium. Sabalenka cleaned up her act with just six unforced errors in the second half of the match, fending off the only break point opportunity against her as she put up 13 winners. The Wimbledon semi-finalist, who won titles in Madrid and Abu Dhabi this year, conceded after the match that she "was struggling" with a few elements of her game and gave credit to Mertens. "It"s always tough matches against her," said Sabalenka.