Cincinnati - Roger Federer began his US Open preparation with a comprehensive 6-4, 6-4 defeat of Peter Gojowczyk in a first ATP Cincinnati Masters appearance since 2015.
The 20-time Grand Slam champion who turned 37 last week, is pacing his tennis year to maintain the longevity of his career, playing a limited schedule and missing events when he feels the need to rest.
He has missed the last two editions through injuries.
The defeat of the 47th-ranked German began his run for a possible eighth title at the tournament. It took just 72 minutes.
The contest was the first for the Swiss since losing to Kevin Anderson in five sets in the Wimbledon quarter-finals on July 11.
Eighth seed Dominic Thiem was forced to withdraw without hitting a ball due to illness.
The Austrian has played just one losing match last week in Toronto on the hardcourts since making a late switch from post-Wimbledon clay.
Nick Kyrgios served 39 aces, including one to save match point, to power into the second round with a dramatic 6-7 (2/7), 7-5, 7-6 (11/9) win over Denis Kudla.
Last year's losing finalist finally saw off the American on his sixth match point.
The Australian will next face Croatian Borna Coric, who beat Daniil Medvedev 6-2, 6-3.
But Kyrgios admitted that his chronic hip problems are starting to become a worry.
"This year has been tough. I started the year very well. Then obviously I hurt my elbow. Then I had an ongoing hip injury," he said.
"We have been definitely thinking about the options with my hip. There is only so much you can do before you have to, I guess, get surgery or something like that.
"Right now I'm just managing it. Obviously I'm feeling my right knee because I'm favouring my left side of my body a little bit.
"I'm just trying to get through it. The physio has been helping me. I've been doing rehab every day. I'm doing everything I can.
"As long as I can serve, I have a good chance to win still."
The 23-year-old has struggled since Wimbledon, retiring from his second-round match against Cameron Norrie in Atlanta before losing at the first hurdle to Stan Wawrinka in last week's Rogers Cup.
But the world number 18 dug deep against Kudla after losing a first-set tie-break, saving three break points in game 11 of the second set, before levelling the match on his third set point and winning a thrilling deciding breaker.
Weekend Toronto finalist Stefanos Tsitsipas ran out of gas in a 7-5, 6-3 loss to Belgian 11th seed David Goffin.
South Korea's Chung Hyeon rallied past a struggling Jack Sock for a 2-6, 6-1, 6-2 victory.
Chung handed the slumping American an eighth straight defeat, with Sock's last win coming in the opening round at Rome three months ago.
Unseeded Milos Raonic advanced past Serb Dusan Lajovic 6-3, 6-3 at the combined ATP and WTA event, the last major tune-up prior to the US Open on August 27.
In the women's draw, Czech ninth seed Karolina Pliskova made a personal breakthrough as she beat Agnieszka Radwanska for the first time after seven previous straight-sets losses to the Pole, winning 6-3, 6-3.
"It was a tough round for me because I've never beaten her, never even taken a set," said Pliskova. "I was so happy when I won the first set because I thought, 'Finally, one set for me'."
Australian 16th seed Ashleigh Barty defeated Czech teenager Marketa Vondrousova 6-3, 7-5, while 14th-ranked Elise Mertens topped Magdalena Rybarikova 6-4, 6-2.
In the second round, French players advanced with Kristina Mladenovic beating Viktoria Kuzmova 6-3, 6-0 while sixth seed Caroline Garcia put out former champion Victoria Azarenka 6-4, 7-5.
Results from the ATP and WTA Cincinnati Masters tournament on Tuesday (x denotes seeding):
Leonardo Mayer (ARG) bt Lucas Pouille (FRA x16) 7-6 (9/7), 6-4
Roger Federer (SUI x2) bt Peter Gojowczyk (GER) 6-4, 6-4
Denis Shapovalov (CAN) bt Kyle Edmund (GBR x14) 6-4, 7-5
Milos Raonic (CAN) bt Dusan Lajovic (SRB) 6-3, 6-3
Robin Haase (NED) bt Filip Krajinovic (SRB) 4-6, 6-2, 6-3
Karen Khachanov (RUS) bt Albert Ramos-Vinolas (ESP) 2-6, 6-3, 6-2
Marius Copil (ROU) bt Philipp Kohlschreiber (GER) 6-4, 6-2
Nick Kyrgios (AUS x15) bt Denis Kudla (USA) 6-7 (2/7), 7-5, 7-6 (11/9)
Borna Coric (CRO) bt Daniil Medvedev (RUS) 6-2, 6-3
Chung Hyeon (KOR) bt Jack Sock (USA) 2-6, 6-1, 6-2
David Goffin (BEL x11) bt Stefanos Tsitsipas (GRE) 7-5, 6-3
Marton Fucsovics (HUN) bt Hubert Hurkacz (POL) 4-6, 6-3, 7-6 (8/6)
Mischa Zverev (GER) bt Damir Dzumhur (BIH) 6-3, 6-4
Kristina Mladenovic (FRA) bt Viktoria Kuzmova (SVK) 6-3, 6-0
Caroline Garcia (FRA x6) bt Victoria Azarenka (BLR) 6-4, 7-5
Petra Kvitova (CZE x8) bt Serena Williams (USA) 6-3, 2-6, 6-3
Elina Svitolina (UKR x5) bt Svetlana Kuznetsova (RUS) 7-6 (7/1), 4-6, 6-4
Kaia Kanepi (EST) bt Varvara Lepchenko (USA) 2-6, 6-3, 6-1
Ashleigh Barty (AUS x16) bt Marketa Vondrousova (CZE) 6-3, 7-5
Ekaterina Makarova (RUS) bt Ana Bogdan (ROU) 6-3, 6-2
Anastasia Pavlyuchenkova (RUS) bt Aleksandra Krunic (SRB) 6-1, 6-0
Camila Giorgi (ITA) bt Anastasija Sevastova (LAT) 6-2, 6-3
Karolina Pliskova (CZE x9) bt Agnieszka Radwanska (POL) 6-3, 6-3
Elise Mertens (BEL x15) bt Magdalena Rybarikova (SVK) 6-4, 6-2
Tatjana Maria (GER) bt Alexandra Kiick (USA) 5-7, 6-3, 6-3
Amanda Anisimova (USA) bt Timea Babos (HUN) 6-3, 6-4
Maria Sakkari (GRE) bt Naomi Osaka (JPN) 6-3, 7-6 (10/8)
Kiki Bertens (NED) bt CoCo Vandeweghe (USA) 6-2, 6-0
Petra Martic (CRO) bt Darya Kasatkina (RUS x12) 4-6, 6-4, 6-3