Qinwen Zheng has been in a poor form recently, losing many matches to much lower ranked players. She advanced to the Round of 16 after scrappy victories over Azarenka and Lulu Sun, so quite weak opposition.
Marta Kostyuk played fairly well in her recent matches. She is a great athlete and she dominated last game against Dolehide physically. It was clear to be seen that she was the stronger and better player.
Statistically, Year to Date, Qinwen Zheng has 60.3% points won on serve and 39.3% on return, while Marta Kostyuk 57.9% on serve and 44.2% on return. So, Marta's stats are better and visually she seems to be the better player, so for me she is the favorite to win this one.