MASON, Ohio – Defending champion Serena Williams initially opted to skip Cincinnati, figuring she’d be just finishing up in Rio de Janeiro. An early upset changed her plans and got her back in the Western & Southern Open with more than just another trophy at stake.

Her No. 1 ranking is on the line, too.

Williams’ attempt to hold onto the top spot will be one of the main intrigues in the tournament that opened on Sunday with a couple of matches scheduled in the men’s bracket. The men’s draw is missing its two most notable plots – Roger Federer trying for an unprecedented eighth Cincinnati title, and Novak Djokovic trying to finally win his first. Both are sidelined by injuries and will have to wait another year for the Masters event.

Meanwhile, Williams wants to get over her loss to 21-year-old Elina Svitolina in the third round at the Olympics. She accepted a wild-card berth to get some matches in Cincinnati, where she has won the title each of the last two years.

Williams heads into the tournament ranked No. 1 for the 306th week overall in her career and her 183rd in a row. It’s the second-longest streak in WTA history, trailing Steffi Graf’s 186 consecutive weeks. She’s been atop the rankings since July 2002 and, at age 34, is the oldest woman to hold the top spot since the computer rankings were introduced in 1975.

Second-ranked Angelique Kerber will take over the top spot if she wins the tournament and Williams loses before the quarterfinals. Williams has a first-round bye and could get a second-round rematch with Svitolina, who opens the tournament against Christina McHale.

The men’s bracket is wide open because of a couple of injuries and travel challenges from the Olympics.

Cincinnati has become Djokovic’s annual quest. It’s the only one he needs to win in order to become the first get all nine current ATP Masters championships, something he’d love to accomplish. He reached the finals last year and lost to Federer, who is 7-0 in title matches. By contrast, Djokovic is 0-5 in Cincinnati finals, never winning even a set.

Djokovic dropped out because of a sore left wrist after he lost at the Olympics. Federer is taking the rest of the year off to rebuild a surgically repaired knee.

Andy Murray enters the tournament as its top seed – he’s won it twice – and a trip north after playing for the gold medal in Rio de Janeiro on Sunday night. Rafael Nadal also will be making the trek after losing the bronze medal match on Sunday to Japan’s Kei Nishikori.

Djokovic, Federer, Murray and Nadal have won 54 of the last 58 Masters events. Djokovic has won four of them this season, with Nadal and Murray winning one apiece. The injuries and the overlapping tournament schedules will open the way for someone else to reach the finals.

Rain jumbled the last two days of qualifying, pushing many qualifying matches over to Sunday. They were again delayed by rain the morning and afternoon.