Roberto Bautista Agut will not improve upon being runner-up at last year's Shanghai Masters after he was eliminated from the first round by Chung Hyeon.
Ninth seed Bautista Agut was beaten in the 2016 final by Andy Murray but fell at the first hurdle this time around in a surprise 6-4 6-3 defeat to the South Korean.
The Spaniard could only engineer four break points throughout the match, with Chung winning the final four games in succession to come out on top.
Juan Martin del Potro, another former finalist in Shanghai, was taken the distance by Nikoloz Basilashvili, while Albert Ramos Vinolas, Richard Gasquet and Andrey Rublev all required three sets to secure victory on Monday.
FINALS BLOW FOR BAUTISTA AGUT
Failing to match his haul of ranking points from making the 2016 final will damage Bautista Agut's hopes of earning a first appearance at the season-ending ATP World Tour Finals.
At world number 13, Bautista Agut represents the joint-biggest win of Chung's career and takes him ahead of Daniil Medvedev in the Race to Milan for the Next Generation ATP Finals.
Chung - ranked 60th in the world - will take on Gasquet in the second round after the Frenchman came from a set down to defeat Robin Haase 6-7 (11-13) 7-5 6-2.
DEL POTRO SURVIVES A SCARE
With the third set level at 4-4, Del Potro came under fire on serve against Basilashvili.
Del Potro - seeded 16th - faced four break points as the Georgian qualifier attempted to gain the chance to serve out the set.
But after six deuces the Argentine finally took the game at the third opportunity, though he still required all three of his match points to set up a meeting with Rublev.
Having suffered a dip in the second set, the Russian romped to a 6-1 4-6 6-1 win over Damir Dzumhur.
NEW GROUND FOR TSITSIPAS
By upsetting Karen Khachanov in the first round, 19-year-old Stefanos Tsitsipas secured his first career win at an ATP Masters 1000 event.
There was also a win for another promising prospect in Frances Tiafoe, who beat Benoit Paire 6-4 6-4.
There were also victories for Feliciano Lopez, Kyle Edmund and Wu Di.