Washington, Sep 6 : Austrian second seed Dominic Thiem saw a fightback from 2014 champion Marin Cilic to go through to the round of 16 of the US Open on Sunday. Thiem was dominant in the first two sets before Cilic finally found his bearings at the Arthur Ashe Stadium. Thiem hit 18 winners, committed just four unforced errors and broke serve four times on seven opportunities in the first two sets. Cilic redoubled his efforts in the third set and managed to put a dent in Thiem's armour. He broke for 4-2 and later saved two break points while serving at 5-3 to force a fourth set. The Croatian threatened to build his momentum in the fourth but Thiem saved three break points to hold for 3-2 before breaking critically for 4-2 in the next game. "He came out a little bit slow and I took all my chances, basically," Thiem said after the match. "I was returning well". The Austrian said he "felt a bit flat" in the third set. "Normally the crowd helps you with these moments. I was two break points down and played a little bit all-or-nothing and it paid off, luckily. Somehow I got my energy back and broke in the next game," he said. Thiem now faces 20-year-old Canadian Felix Auger Aliassime in his last 16 match. /IANS