Sock sunk by Robredo
Tommy Robredo saw off Jack Sock in straight sets to book a second round clash with another American, Sam Querrey, at the ATP Cincinnati Masters.
The Spaniard - a two-time semi-finalist at the tournament - impressively coped with his opponent's heavy serve to triumph 7-5 (7/5) 6-3 after one hour and 38 minutes on Center Court.
Robredo, seeded 16, had to come through a tense tie break to take the opening set with Sock, who was restricted to just three aces and also needed treatment for an arm injury during the match.
In other first-round matches to take place on Monday, Frenchman Gael Monfils needed three sets to get past world no 63 Federico Delbonis, eventually triumphing 6-3 3-6 6-3.
Philipp Kohlschreiber also progressed in three, coming out on top 6-3 4-6 6-4 to record his maiden win over Frenchman Jeremy Chardy, while fellow German Benjamin Becker finally got the better of Ivo Karlovic at the seventh time of asking.
Becker won 7-6 (7/5) 6-4 against the big-serving Croatian, with his reward being a clash with third seed Stan Wawrinka in the next round.
Lu Yen-Hsun advanced with a 6-3 6-4 win Denis Istomin and qualifier Marinko Matosevic progressed with a 6-4 7-6 (7/4) result over Nicolas Mahut of France. In an all-Spanish tie, Fernando Verdasco battled back from a set down to eliminate compatriot Marcel Granollers, 5-7 6-3 7-6 (7/5).
Meanwhile, Andy Murray now knows he will take on Joao Sousa in his opening match after the man from Portugal knocked out American Chase Buchanan 5-7 7-6 (7/3) 7-6 (7/5).