Ohio - Andy Murray admitted on Monday he doesn't know how he'll perform in his US Open title defence, but he hopes a solid showing in Cincinnati this week will help him prepare.
The world number two from Scotland, who ended Britain's 77-year wait for a Wimbledon men's champion last month, said that despite winning two Grand Slams he faces a new challenge when defending the American title.
"A lot of the pressure comes from yourself, how much you want to keep doing and how much you want to keep working," Murray said.
"I hope there's still pressure there and I hope there's still some expectation. There will be nerves and stuff.
"It's going to be a new experience for me, though, at the US Open. I've never defended a Grand Slam title before, and hopefully I'll deal with that OK."
The US Open begins on August 26. Murray meanwhile has another practice day in Cincinnati as he waits to face Mikhail Youzhny or Ernests Gulbis - the Latvian who beat him last week in the second round of the Montreal Masters.
In first-round matches of the combined WTA and ATP Masters tournament in Cincinnati on Monday, German Philipp Kohlschreiber defeated former finalist Mardy Fish 7-5, 6-2 to set up a Tuesday night second-round match with five-time champion Roger Federer.
Swiss great Federer, who will be playing on hardcourts for the first time since March, said he is over the back injury which bothered him on clay last month and is ready to bid for another Cincinnati crown.
The omens are good, at least - Federer has a 7-0 record over his German friend Kohlschreiber.
Spanish 13th seed Nicolas Almagro fell in a Masters first round for the second week in succession, going down to Bulgarian Grigor Dimitrov 7-6 (7/3), 6-4. Dimitrov fired 19 aces.
Czech Radek Stepanek knocked out Italian 14th seed Fabio Fognini 6-2, 6-4 while Serb Janko Tipsarevic sent American Sam Querrey packing 6-7 (5/7), 6-3, 6-4.
Brian Baker, who retired at the Australian Open with a serious knee injury and has been in rehab ever since, opened with a 7-5, 6-3 defeat of Denis Istomin, four days after the Uzbek took a set off world number one Novak Djokovic in Montreal.
Argentina's Juan Monaco kept Austrian Jurgen Melzer winless since late June at Wimbledon with a 6-3, 6-2 victory, while Spain's Marcel Granollers beat Lukas Rosol 5-7, 6-4, 6-0. Jeremy Chardy won an all-French battle over Adrian Mannarino 6-4, 6-3.
In the women's draw, ninth seed Angelique Kerber, beaten in her opening match last week in Toronto, opened with a 6-7 (6/8), 6-0, 6-1 win over Italian Karin Knapp.
American Venus Williams, a first-round casualty in Toronto last week, bounced back with a 6-4, 6-1 victory over Slovakian qualifier Jana Cepelova.
"In Toronto I was really nervous about serving," said Williams, who has been recovering from a back injury that saw her miss Wimbledon. "Tonight, I wasn't afraid of hurting myself."
Australian Samantha Stosur, the 11th seed, continued the improvement in her hard-court game with a 6-1, 7-5 first-round defeat of fellow grand slam winner Svetlana Kuznetsova. Russian 16th seed Maria Kirilenko beat Anna Tatishvili 4-6, 7-6 (7/4), 7-6 (7/4).
The joint event features the elite of both tours, with Toronto champion Serena Williams heading a women's field that includes all top-20 players.
World number three Maria Sharapova will be playing for the first time since a Wimbledon second-round defeat and subsequent healing of a hip injury.