Andy Murray needs to "step up" at the US Open, according to John McEnroe.
Simona Halep suffered a blow with the US Open just around the corner after being dumped out of the Connecticut Open against world number 68 Magdalena Rybarikova.
Top seed John Isner booked his place in the third round of the Winston-Salem Open with a straight-sets victory over Bradley Klahn.
British men's number one Andy Murray has been seeded eighth for this year's US Open.
Defending US Open champion Rafael Nadal is to miss this year's tournament because of injury.
Roger Federer defeated David Ferrer in the final of the Western & Southern Open in Cincinnati on Sunday.
Caroline Wozniacki got off to a slow start at one of her favourite tournaments but eventually came through to reach the second round of the Connecticut Open.
Bradley Klahn beat fellow American Noah Rubin to set up a second-round encounter against top seed John Isner in the Winston-Salem Open.
Roger Federer remains on course for his sixth Western & Southern Open title after advancing to the final with victory over Milos Raonic.
World number one Novak Djokovic's preparations for the US Open suffered another blow after a straight-sets defeat to Tommy Robredo in the third round of the Western & Southern Open.
Second seed Roger Federer showed flashes of his absolute best with a clinical straight sets victory over Andy Murray in the quarter-finals of the Western & Southern Open.
Roger Federer's quest for a sixth Western & Southern Open got off to an unconvincing start as he needed three sets to get past Vasek Pospisil in Cincinnati.
Andy Murray will begin his 2015 season at the Hopman Cup in Perth alongside fellow British number one Heather Watson.
Novak Djokovic had to dig deep to fend off Frenchman Gilles Simon in the second round of Cincinnati's Western & Southern Open.
Neither Ukraine nor Israel will be allowed to play next month's Davis Cup ties on home soil because of security issues, the International Tennis Federation has ruled.