Moscow - Reigning champion Damir Dzumhur saw his Kremlin Cup title defence come to an abrupt halt on Tuesday with a first-round loss to Belarusian qualifier Egor Gerasimov.
The Bosnian, who won both the St Petersburg and Moscow tournaments last year, slumped to a 6-1 6-7 (5/7), 6-2 loss to the world number 217.
Gerasimov will next face charismatic Frenchman Benoit Paire for a place in the last eight.
Paire, who has not reached a quarter-final since February, beat Mischa Zverev 7-6 (8/6), 2-6, 6-1.
"It was a good match. I wasn't doing well at the beginning but then I got much better," Paire said.
In the women's draw, former world number two Vera Zvonareva set up a second-round meeting with second seed Karolina Pliskova.
The 34-year-old Zvonareva, a two-time Grand Slam finalist, saw off fellow Russian qualifier Irina Khromacheva 6-3, 6-3.
Top seed Simona Halep, who secured the year-end world-number-one ranking on Monday, pulled out of the tournament earlier on Tuesday to rest a troublesome back injury.
"While it's disappointing to withdraw, I know it's important to put my health first," she said.