Alexander Zverev will not compete in defending champion Germany's United Cup quarter-final against Kazakhstan due to a biceps strain.
The No. 2 player in the PIF ATP Rankings will be replaced in singles by Daniel Masur. The World No. 254 will face Alexander Shevchenko in the pair's first Lexus ATP Head2Head meeting.
Zverev won both of his singles matches in Group E action, defeating Thiago Monteiro 6-4, 6-4 and Zhang Zhizhen 2-6, 6-0, 6-2. The two-time Nitto ATP Finals champion played twice on Tuesday against China, also partnering Laura Siegemund to a mixed doubles victory.
Masur has climbed as high as World No. 176 and is a two-time ATP Challenger Tour champion. The German earned his lone tour-level win in 2018 at Hamburg, where he defeated Maximilian Marterer before falling to Diego Schwartzman.
Elena Rybakina defeated Siegemund 6-3, 6-1 in the first match of the Germany-Kazakhstan quarter-final tie.
