World number six Kei Nishikori suffered a surprise defeat to Jack Sock in the second round of the Shanghai Masters.
The US Open finalist lost in straight sets to his unseeded American opponent following two close-fought sets.
Neither player had a single break point opportunity as the first set went the distance, but Sock, who is ranked 60 in the world, took two points off Nishikori's serve to win the breaker by the smallest possible margin.
The second set took on a similar pattern, with the respective servers holding to love in the first three games, but a tiebreak was not needed this time as Sock clinched a 7-6[5] 6-4 win with the first break of the match.
The 22-year-old, who won the men's doubles at Wimbledon this year alongside Vasek Pospisil, will face Julien Benneteau in the last 16 after the experienced Frenchman upset Grigor Dimitrov in China.
