Top seeds Novak Djokovic and Roger Federer cruised into the second round of the Cincinnati Masters as Serena Williams withdrew from the tournament with a back injury.
Serbian world number one Djokovic enjoyed a comfortable 7-5, 6-1 victory against American Sam Querrey at the Lindner Family Tennis Center.
Rain held up Swiss star Federer's assault on the Association of Tennis Professionals World Tour Masters 1000 event before he progressed with a 6-3, 6-4 win against Juan Ignacio Londero of Argentina.
Rogers Cup finalist Daniil Medvedev of Russia got his Cincinnati Masters campaign off to a winning start with a 6-2, 7-5 triumph against Briton Kyle Edmund.
Switzerland's Stanislas Wawrinka fought back from losing the first set against Bulgarian Grigor Dimitrov before coming through to win 5-7, 6-4, 7-6.
American Williams announced her withdrawal from the Women's Tennis Association Premier 5 event as she continues to struggle with back spasms.
The world number eight withdrew from last weeks Rogers Cup final with the same issue having trailed Canadian teenager Bianca Andreescu in the first set in Toronto.
In the second round, Madison Keys from the United States won 6-7, 7-6, 6-4 against Spaniard Garbiñe Muguruza.
Defending champion Kiki Bertens from The Netherlands was knocked out by American Venus Williams in a final set tie-break.
Williams progressed at Bertens' expense courtesy of a 3-6, 7-6, 6-7 victory.
Croatian Donna Vekić won 6-4, 6-3 against Kateřina Siniaková of the Czech Republic and Russian Maria Sharapova progressed with a 6-3, 7-6 victory over American Alison Riske.