MASON, Ohio (AP) Angelique Kerber has a chance to become No. 1 at the Western & Southern Open. The men's bracket is wide open, too, providing an opportunity for someone outside the Big Four to claim a Masters title.
It's a week for the up-and-comers.
Two-time defending champion Serena Williams pulled out of the tournament with an inflamed shoulder, making it possible for Kerber to overtake her in the rankings. She would move up to No. 1 by winning the tournament for the first time.
Williams has been in the top spot for 183 consecutive weeks, the second-longest such stretch in WTA history.
''There's a long way to go,'' Kerber said on Tuesday. ''I'm not thinking about that. Somebody told me about that, but I'm trying not to listen.
''Everyone dreams about being No. 1. It would be amazing. I would give everything to reach that goal.''
The men's bracket provides an opportunity for second-tier players to move up for a week. Defending champion Roger Federer and top-ranked Novak Djokovic are sidelined by injuries, and Andy Murray and Rafael Nadal are still recovering from hectic weeks at the Olympics in Rio de Janeiro. The four of them have accounted for 54 of the past 58 Masters titles.
There's a chance for somebody else to get a Masters trophy on Sunday.
''I played 23 hours in seven days,'' Nadal said on Tuesday, referring to his Olympics stay. ''My body is a little bit tired. Obviously it's going to be tough.''
Rain repeatedly interrupted the first full day of matches on Tuesday. Stan Wawrinka - one of those who could benefit from the open men's bracket - lost the first set to wild card Jared Donaldson 6-2, but led the second set 3-0 when play was suspended for the night because of rain. John Isner overcame two delays while beating Fabio Fognini 6-3, 6-2, aided by the Italian's four double faults in the second set.
Kerber opens play on Wednesday, trying to extend a successful season that includes the Australian Open championship, a Wimbledon final, and a silver medal in Rio.
''That was a special week,'' Kerber said of the Olympics. ''I had a lot of experiences. The atmosphere was different than a normal tournament. I had a great week. I won a medal. That was my goal.''
Nadal had a lot of fun in Rio, but came away running on fumes.
The 30-year-old Spaniard missed two months with an injured left wrist and returned in Rio, where he reached the semifinals in singles - a good showing given how much time he was away from the court. He's hoping to get in more matches in Cincinnati to get ready for the U.S. Open without aggravating the wrist.
''It was not an easy injury and it still bothers me a little bit,'' Nadal said.
Murray beat Juan Martin del Potro in a wild, four-set match for his second Olympic gold medal on Sunday that left him drained. Then came the flight directly to Cincinnati - yes, he brought his medal along.
''The match against Juan Martin was so up and down, so emotional, physical,'' Murray said. ''It was a really, really hard match, one of the hardest matches I've played.
''Doing the long trip straight afterward was tough. Hopefully it will be able to rain in the next few days.''
AP freelance writer Mark Schmetzer contributed to this report.