World number one Novak Djokovic got his US Open build up off to a smooth start with a straight-sets win in Cincinnati on Tuesday as back trouble forced Serena Williams to withdraw.

Roger Federer beat Argentine Juan Ignacio Londero 6-3, 6-4 to win a rain-interrupted opening match. The Swiss, a seven-time champion in Ohio, last played a month ago when he lost the Wimbledon final to Djokovic.

But he showed no sign of rust and after the weather forced a one-hour halt with Federer up a set and level at 2-all in the second the 20-time Grand Slam champion came back out to finish his job on a second match point.

Defending champion Djokovic double faulted three times as he dropped serve in the opening game against Sam Querrey but quickly put things right and pulled away for a 7-5, 6-1 victory over the American.

“I ended the match well even if the start was nervy,” said Djokovic, who would break Querrey twice to take the first set then roll to victory in his first match since beating Roger Federer in the Wimbledon final last month.

“Sam was feeling comfortable on the court at the start and dictating. It was tough facing his big serve,” Djokovic said.

“I’m just pleased with this challenge in quick conditions, I’m glad I overcame it. “I’m hoping for a better performance in the next round.”

Williams, meanwhile, was left just trying to get healthy before the August 26 start of the US Open, pulling out of her scheduled first-round match against qualifier Zarina Diyas just days after she was forced to retire from the WTA Toronto final with back spasms.

“I came to Mason (Ohio) on Sunday and have tried everything to be ready to play tonight, and was still hopeful after my practice this morning,” Williams said in a statement released by tournament officials. “But unfortunately my back is still not right.”

It’s another injury blow for 23-time Grand Slam champion Williams, who was hampered by knee trouble earlier this season and remains in search of her first title since the 2017 Australian Open.

In early action, Stan Wawrinka won a first-round battle as he eliminated 2017 Cincinnati champion Grigor Dimitrov 5-7, 6-4, 7-6 (7/4).

Under leaden skies after an overnight thunderstorm, Dimitrov broke in the 12th game of the first set to take the lead.

The 33-year-old Swiss fought back with the only break in the second set. Wawrinka then raced into a 5-1 lead in the final set but wasted two match points in the eighth game and was broken twice while serving for victory.

Wawrinka had to hold off his Bulgarian opponent in the final-set tiebreaker before squeezing through in more than two and a half hours with his 10th ace.

Wawrinka, who played a 2012 semi-final here, will face Russian qualifier Andrey Rublev, a 6-7 (4/7), 6-4, 6-2 winner over 15th seed Nikoloz Basilashvili from Georgia.

In other opening-round matches, Montreal semi-finalist Gael Monfils exited to Frances Tiafoe 7-6 (9/7), 6-3 and Belgian David Goffin beat Taylor Fritz of the United States 6-4, 4-6, 6-4.

Venus topples Bertens

In women’s play, defending champion Kiki Bertens fell in the second round to Venus Williams, with the 39-year-old American putting her decades of experience to good use in a 6-3, 3-6, 7-6 (7/4) victory.

“Before the match I felt quite good. Stepping on court, I’d say first few games was pretty bad,” Bertens said.

“I didn’t start the match in a loose way. I knew that I had to defend the title.”

Williams, who reached the semi-finals in 2012, has not been past the second round in Cincy since then.

Former US Open finalist Madison Keys knocked out 2017 Cincinnati champion Garbine Muguruza of Spain 6-7 (4/7), 7-6 (7/5), 6-4.

Former number one Angelique Kerber continued a dismal hardcourt summer, losing in the first round for a second straight week as she fell 7-6 (9/7), 6-2 to Estonian Anett Kontaveit.