Aryna Sabalenka reacts during a match. File

Sabalenka eased past Greece's Maria Sakkari 6-0, 6-1 in her group-stage opener on Sunday but that didn't prevent her from taking aim at the organisation.

"I have to say that I am very disappointed with the WTA and the experience so far at the WTA Finals," the 25-year-old from Belarus wrote on Instagram.

"As a player I feel disrespected by the WTA. I think most of us do. This is not the level of organisation we expect for the Finals. "To be honest I don't feel safe moving on this court a lot of the time."

Sabalenka claimed players were not allowed to practice on the court until the day before the opening. "It's just not acceptable to me with so much on the line and with so much at stake."

Sabalenka said her criticism of the event was aimed at the WTA, not local organisers. "I definitely want to show my appreciation for the local tournament organisers, everyone that built the court at the last minute, and everyone that's working here at the event," she wrote. "I know it's not their fault."

American Jessica Pegula defeated Kazakhstan's Elena Rybakina 7-5, 6-2 in 83 minutes in the other singles group stage match at the $9 million outdoor hardcourt showdown.

Sinner defeated Medvedev for the second time in three weeks, having also overcome the Russian in the Beijing final at the start of October.

Medvedev was attempting to retain the title he won last year in Austria but Sinner nudged ahead in the final set, taking his ninth break point of an 18-minute game for a 3-1 lead.

Former world number one Medvedev broke back the following game only to immediately again drop serve, allowing Sinner to close out victory in just over three hours.

Both players are in the draw for next week's Paris Masters. Sinner and Medvedev have already qualified for the ATP Finals in Turin.