Zverev and Cilic up and running as Sock shocked at Flushing Meadows
Alexander Zverev began his quest to make a major breakthrough at the US Open with a tough victory over qualifier Darian King, while Marin Cilic is also up and running in New York.
With Novak Djokovic and Andy Murray absent from the year's final grand slam, Roger Federer struggling with injury and Rafael Nadal's form on the hard courts patchy, fourth seed Zverev is being tipped for glory in the Big Apple.
And the German showed his battling qualities in a first round that produced wins for 2014 champion Marin Cilic and Jo-Wilfried Tsonga, but saw home hope Jack Sock bite the dust.
LATE FINISH FOR BATTLING ZVEREV
It was not the most straight-forward of outings for Zverev, who has won titles in Washington and Montreal in the build-up, but he eventually prevailed 7-6 (11-9) 7-5 6-4 against a plucky King.
Zverev, who has never gone beyond round two in New York, chalked up 62 errors but showed his composure in a match that went beyond 2am local time.
Borna Coric is next up for the German.
CILIC OFF TO WINNING START
Cilic, a finalist at Wimbledon this year, returned to the location of his greatest triumph and overcame American Tennys Sandgren 6-4 6-3 3-6 6-3.
Seeded fifth for the tournament, Cilic showed little sign of the adductor injury that sidelined him for the Montreal and Cincinnati Masters as he hit 55 winners and 11 aces in two hours, 47 minutes.
"Everything feels okay physically," said Cilic, who will face Florian Mayer in the second round. "I'm very happy with that. That's the number one thing. It was a good fight towards the end of the match. Also I was not running away to try to shorten up the points. I was actually feeling that could be also quite good for me to get in some rallies, battle it out, just mentally-wise to get myself in a good position."
Seeds John Isner, Sam Querrey, Gilles Muller, Kevin Anderson, Lucas Pouille, Albert Ramos-Vinolas, Pablo Carreno Busta and Diego Schwartzman also progressed.
However, two-time semi-finalist David Ferrer (27) was beaten in four sets by Mikhail Kukushkin and Robin Haase (32) fell to Britain's Kyle Edmund.
THOMPSON STUNS SOCK IN MARATHON
It took three hours, 56 minutes but Aussie Jordan Thompson prevailed 6-2 7-6 (14-12)1-6 5-7 6-4 against 13th seed Sock.
Sock rallied from 5-4 down and saved two matches points to force a deciding fifth set.
But Thompson - who claimed his first win at Flushing Meadows - regained his composure to trump Sock for the second time this year, having shocked the American during the Davis Cup in Brisbane.
TSONGA CRUISES THROUGH
French eighth seed Tsonga made light work of Marius Copil 6-3 6-3 6-4.
At a jampacked Court 17, three-time US Open quarter-finalist Tsonga was a class above as he hit 29 winners against the Romanian.
Following the withdrawal of second seed Andy Murray, the draw has opened up for Tsonga, who next plays Canadian sensation Denis Shapovalov.
Shapovalov announced himself with wins over Juan Martin del Potro and Rafael Nadal en route to the semi-finals of the Montreal Masters.
And the 18-year-old showcased his talented as he made his US Open debut, defeating Daniil Medvedev 7-5 6-1 6-2.