Zverev won their only previous meeting. That was on an indoor court in Basel in October 2016. He won in straight sets.
Pella took advantage of a poor match from Ramanathan in the last round. He was broken in the opening game but came back to win in straight sets. Pella has good stats this year although bear in mind he plays weaker opponents in Challengers. He
did play well in Miami in March. He made the third round beating Dimitrov in straight sets.
Zverev started the year strongly and got to the quarter finals at the Australian Open. After that his form dipped for a few months. He found his form again in May and reached the final in Geneva on clay losing to Wawrinka in 3 sets in the final. He had a semi final on clay in Stuttgart and reached the third round at Wimbledon.
I have to favour Zverev to win this. Wait for 1.75 before backing him. He starts around 1.55.