Ohio - ATP Cincinnati Masters second seed Novak Djokovic says he has to remind himself which continent he is on as his fast-paced schedule spins swiftly with the Olympics done and the US Open near.
"If you don't keep track of time and places where you are, it happens - sometimes you wake up and say, 'OK, which time zone am I in right now?,'" Djokovic said.
Djokovic made the quick-change from Wimbledon Olympic grass to Toronto hardcourts last week to win Canada's Masters event.
"It's the way it is for everybody, mostly for top players," he said. "We are aware of the fact that Olympic Games are going to make our schedule more difficult and busier than previous years.
"That's what happens always in the tennis schedule when you have Olympic year coming up. We all tried to work our schedule around that. For now, I'm feeling good."
Djokovic will play his opening match on Wednesday against Italian Andreas Seppi, with top seed Roger Federer and Olympic gold medalist Andy Murray also opening then.
In Tuesday first-round matches, US 10th seed Mardy Fish ousted Spain's Feliciano Lopez 6-2, 6-3 while longtime friend and two-time tournament champion Andy Roddick lost to Frenchman Jeremy Chardy 7-6 (7/4), 6-3.
"I felt fine early on, and then I had one lunge forehand and the back kind of goes out or spasms a little bit. It got progressively worse," Roddick said. "I feel a lot better about where my game is at now -- today was just one of those things, what are you going to do about it?"
German Tommy Haas continued his fighting return to top form by outlasting Argentine David Nalbandian 6-7 (0/7), 7-6 (7/4), 6-3 in a match lasting almost 3 1/2 hours.
Haas won the Halle title in June on grass over Federer and played in finals at Hamburg and Washington. He reached the quarter-finals last week in Toronto and has lined up a Cincinnati date with Olympic bronze medalist Juan Martin del Potro, the eighth seed.
Japan's 14th seed Kei Nishikori opened with a victory over Marcel Granollers 6-1, 6-2.
In women's play, unseeded Olympic doubles champion Venus Williams found herself in a struggle with Russian Maria Kirilenko, the 12th seed, before pulling out a 6-3, 6-7 (5-7), 6-2 first-round win in nearly 2 1/2 hours.
The 32-year-old American was helped by 11 aces but did not do herself any favors as she failed to serve out a straight-sets win.
"I think I was putting too much emphasis on my serve," Williams said. "I wanted to win it off the serve, and then when I had to hit some groundstrokes I just wasn't patient enough.
"I should have stuck in there and worked the point and hit a lot of balls and be ready to win the point. I got a little impatient, and that doesn't work, especially against a player like her. I changed my ways in the third."
Williams was joined with a sloppy win by younger sister Serena as the second seed dispatched Eleni Daniilidou of Greece 6-3, 6-4 for a third-round place despite 44 unforced errors.
The winner said that switching from the Olympic grass to the hardcourts meant an abrupt change.
"It's just a tough turnaround. I think I'll be okay just getting that match under my belt," she said.
"When you're playing on grass and then you go to hardcourt, especially this hardcourt in particular is playing a little slower than other hardcourts."
US Open winner Samantha Stosur made heavy weather of a win over Anabel Medina Garrigues 6-3, 6-7 (6/8), 6-4 while Swede Johanna Larsson upset French eighth seed Marion Bartoli 6-4, 6-2.
Russian Elena Makarova eliminated Nadia Petrova when her 15th-seeded compatriot trailed 7-6 (10/8), 3-1.
Results from the second day of the ATP and WTA Cincinnati Masters Series event on Tuesday (x denotes seeding):
Angelique Kerber (GER x5) bt Timea Babos (HUN) 3-6, 6-1, 6-2
Serena Williams (USA x2) bt Eleni Daniilidou (GRE) 6-3, 6-4
Johanna Larsson (SWE) bt Marion Bartoli (FRA x8) 6-4, 6-2
Samantha Stosur (AUS x3) bt Anabel Medina Garrigues (ESP) 6-3, 6-7 (6/8), 6-4
Sloane Stephens (USA) bt Tsvetana Pironkova (BUL) 6-4, 6-1
Anna Tatishvili (GEO) bt Klara Zakopalova (CZE) 7-6 (7/5), 7-5
Venus Williams (USA) bt Maria Kirilenko (RUS x12) 6-3, 6-7 (5/7), 6-2
Andrea Hlavackova (CZE) bt Bethanie Mattek-Sands (USA) 6-3, 7-5
Sofia Arvidsson (SWE) bt Tamira Paszek (AUT) 6-1, 2-1 retired
Sesil Karatantcheva (KAZ) bt Kiki Bertens (NED) 6-2, 6-1
Elena Makarova (RUS) bt Nadia Petrova (RUS x15) 7-6 (10/8), 3-1, retired
Sorona Cirstea (ROM) bt Yvonne Wickmayer (BEL) 6-7 (2/7), 6-2, 6-3
Daniela Hantuchova (SVK) bt Zheng Jie (CHN) 2-6, 7-6 (7/1), 6-3
Roberta Vinci (ITA) bt Madison Keys (USA) 6-3, 6-3
Anastasia Pavlyuchenkova (RUS) bt Carla Suarez Navarro (ESP) 6-4, 6-0
Yaraslava Shvedova (KAZ) bt Lucie Safarova (CZE x16) 6-3, 6-4
Chanelle Scheepers (RSA) bt Christina Mchale (USA) 3-6, 6-3, 3-0, retired
Bernard Tomic (AUS) bt Brian Baker (USA) 6-4, 6-3
Tomas Berdych (CZE x5) bt Yu Yen-hsun (TPE) 6-3, 6-4
Alex Bogomolov (RUS) bt Jarkko Nieminen (FIN) 6-4, 6-3
Kei Nishikori (JPN x14) bt Marcel Granollers (ESP) 6-1, 6-2
Tommy Haas (GER) bt David Nalbandian (ARG) 6-7 (0/7), 7-6 (7/4), 6-3
Jeremy Chardy (FRA) bt Andy Roddick (USA x16) 7-6 (7/4), 6-3
Mardy Fish (USA x10) bt Feliciano Lopez (ESP) 6-2, 6-3
Carlos Berlocq (ARG) bt Santiago Giraldo (COL) 7-5, 6-3
Sam Querrey (USA) bt Jurgen Melzer (AUT) 4-6, 6-2, 6-4
Radek Stepanek (CZE) bt Albert Ramos (ESP) 6-2, 6-2
Marcos Baghdatis (CYP) bt Alejandro Falla (COL) 6-4, 6-3
Milos Raonic (CAN) bt Richard Gasquet (FRA) 7-6 (7/4), 6-3