Novak Djokovic survived a nervous start to see off Sam Querrey 7-5, 6-1 at the Cincinnati Masters on Tuesday as the World No 1 returned to competition for the first time since beating Roger Federer in last month's epic Wimbledon final.
Seven-time Cincinnati champion Federer, also playing his first match since Wimbledon, was quicker to get back in the groove, needing just 61 minutes to dispatch Juan Londero 6-3, 6-4 in their night match, which was halted in the second set due to rain.
Defending champion Djokovic fired his 15th ace on match point to defeat Querrey but his serve had been an issue earlier in their second-round encounter.
"I made three double faults in my opening game and lost the break. I was quite a nervous at the start," he said after falling behind 2-0 to the American.
"I'm really glad to overcome a challenge like this and get myself going."
Djokovic, who defeated Federer in a classic five-set battle at Wimbledon in July, said he had needed time to recover from the mental strain of that match.
"That was one of the most epic matches I've been a part of and probably the most demanding one from the mental perspective," he said.
"So it really did take me some time to recharge my batteries, especially in my head. My body was fine."
Djokovic will face either John Isner or Pablo Carreno Busta in the next round, with a potential quarter-final meeting with Australian Nick Kyrgios.
Federer will take on Swiss compatriot Stan Wawrinka or Andrey Rublev in the third round.
Wawrinka overcame Bulgaria's Grigor Dimitrov 5-7, 6-4, 7-6(4) in a roller-coaster first-round match.
Dimitrov clawed his way back from a double break down in the decider to level at 5-5 and after Wawrinka broke again, Dimitrov forced the tiebreak.
The Swiss surged into a 4-0 lead but Dimitrov rallied once more before Wawrinka closed out the match with an ace.
American Frances Tiafoe beat Frenchman Gael Monfils 7-6(7), 6-3 and Canadian Denis Shapovalov came from behind to see off Portugal's Joao Sousa 2-6, 6-3, 6-2.
Tennis: Venus stuns holder Bertens in Cincinnati, Serena withdraws
Former world number one Venus Williams, playing what she called "smart" tennis, beat defending champion Kiki Bertens 6-3, 3-6, 7-6(4) in the second round of the Cincinnati Masters on Tuesday before her sister Serena withdrew due to back problems.
Serena, who was forced her to retire from the Rogers Cup final on Sunday because of back spasms, had been due to play Zarina Diyas in the first round as she prepares for her tilt at a 24th Grand Slam title at this month's US Open.
After Venus and Bertens split the first two sets, Williams surged ahead 3-0 in the third thanks to some excellent volleying but struggled to put her opponent away, committing numerous errors when serving for the match at 5-3.
Bertens held at 5-5 and then forced a tie-break with an ace.
Williams, however, took control of the tie-break, moving ahead 3-0 and 6-2 before sealing the win..
"I just tried not to go too big, because I can go so big and I have a lot of power and it's not always easy to control it," Williams said.
"So I'm trying to play smart instead of going hog wild, which is extremely easy to do."
German 13th seed Angelique Kerber exited the tournament when she lost to Estonia's Anett Kontaveit 7-6(7), 6-2. The 23-year-old Kontaveit came from 5-3 down and saved a set point at 6-5 in the opener against the former world number one.
She broke Kerber twice to win the second set, winning the first nine points, and two holds to love gave her a 4-0 lead.
Swiss 12th seed Belinda Bencic retired with a foot injury when trailing Victoria Azarenka 6-4 1-0. Former world number one Azarenka will face Donna Vekic in the second round after the Croatian beat Czech Katerina Siniakova 6-4, 6-3.
American Madison Keys beat fellow big hitter Garbine Muguruza of Spain 6-7(4), 7-6(5), 6-4.
Russia's Ekaterina Alexandrova wasted little time in beating Carla Suarez Navarro of Spain 6-4, 6-1 in 56 minutes while China's Zheng Saisai beat Viktoria Kuzmova of Slovakia 6-3, 6-1.