BENGALURU – US Open champion Bianca Andreescu was forced to dig deep in the second set to beat Elise Mertens in straight sets, winning 6-3 7-6(5) to move into the last 16 of the China Open in Beijing on Wednesday.Canadian fifth seed Andreescu, who needed three sets to dispatch her Belgian opponent in the US Open quarter-finals last month, fought back from 4-1 down in the second set and also saved a set point to win in an hour and 55 minutes.Andreescu not only converted five of six break points but also saved eight of 12 she faced. She next faces American qualifier Jennifer Brady who upset 11th seed American Madison Keys 2-6 6-4 6-4.World number one Ash Barty won her 50th match of the calendar year but she was made to toil for a 6-3 6-7(5) 6-2 victory over China’s Zheng Saisai that sealed her spot in the quarter-finals.Barty needed one break of serve to take the opening set but was pushed to the limit in the second. The 39th-ranked Chinese levelled the match by clinching the tiebreak after the top seed made 23 unforced errors in the second set.The Australian struggled to convert break points (3/14 in the match) at crucial moments but managed to win two in the final set to advance to the quarters where she will face seventh seed Petra Kvitova, who beat Belinda Bencic 6-3 6-3.