Djokovic, the world number one and defending US Open champion, had been tested in his first-round match against Poland’s Jerzy Janowicz on Monday. The Serbian, who has been struggling with a wrist injury in recent weeks, dropped the second set before coming through 6-3, 5-7, 6-2, 6-1.

However, he was handed a welcome walkover on Wednesday when Jiri Vesely was forced to retire from the year’s fourth and final Grand Slam due to what organizers said was inflammation to his left forearm.

After a blistering start to the year when he won the majors in Australia and France, Djokovic has been struggling for form and was ousted in the third round at Wimbledon before crashing to a shock first-round exit at the Olympic Games.

Djokovic now moves on to face Russia's Mikhail Youzhny.

Fourth-seeded Rafael Nadal of Spain defeated Italy's Andreas Seppi 6-0, 7-5, 6-1 and is to face Russia's Andrey Kuznetsov in the next round.

During their match, the new retractable roof on the Arthur Ashe Stadium was needed for the first time when light rain began falling in the second set. It too only a few moments for the roof to close before Nadal and Seppi resumed their match beneath it.

Wimbledon finalist Milos Raonic was ousted 6-7 (4), 7-5, 7-5, 6-1 by 120th-ranked qualifier Ryan Harrison of the United States. The fifth-seeded Canadian required on-court medical treatment for problems with his left wrist and left thigh during the match.

Easy ride

The women's second seed, Angelique Kerber, the Australian Open champion, saw off Croatia's Mirjana Lucic-Baroni 6-2, 7-6.

Kerber had had an easy ride in the first round when her Slovenian opponent Polona Hercog retired at 6-0, 1-0 in the German’s favor. The German could take over from Serena Williams as world number one depending on how results go at Flushing Meadows. Kerber is to face Catherine Bellis of the United States, for a place in the last 16.

Also on Wednesday, the third seed, Garbine Muguruza of Spain, was knocked out in the second round in a 7-5, 6-4 upset by 48th-ranked Anastasija Sevastova of Latvia.

Idr,pfd/dv (Reuters, AFP, AP)