DUBAI — Petra Kvitova succumbed to injury on Monday as the Czech star was forced to withdraw from the Dubai Championships, a day after lifting her second title of the year.

The double Wimbledon winner informed organizers that a leg injury would prevent her from playing and deny her the chance for a third successive triumph after back-to-back titles in St Petersburg and Doha.

Kvitova joins five-time Grand Slam champion Maria Sharapova (forearm) and former world number one Simona Halep (ankle) on the casualty list in Dubai.

During her impressive run at last week's Qatar Open, Kvitova upset top-ranked Caroline Wozniacki and reigning Wimbledon champion Garbine Muguruza, in the semifinals and final respectively.

"I am really disappointed to be withdrawing," Kvitova said in a statement. "I spent my off-season in Dubai this year and was really looking forward to being back at a tournament where I have had some great results.

"After playing a lot of tennis in the past few weeks, and winning the title (Sunday), my whole body is in much need of a rest. I can't wait to be back in Dubai next year and wish you all the best this week."

Japan's Naomi Osaka earned the first upset win of the week, knocking out eighth seed Kristina Mladenovic 6-2, 6-2 in 70 minutes with six breaks of serve. Elena Vesnina beat Peng Shuai 6-4, 7-5 after losing her last two matches to the Chinese player.

Vesnina has played Dubai in seven of the last eight years but has never reached the fourth round. The world No. 23 from Russia is eager to see some thoroughbred horse racing in the emirate during her time off court.

"I really want to see the horse riding, I heard that it's really big here. It's one of the best in the world. Massive stadium. Actually, that's one of my dreams, to visit the horse riding, horse racing, to see the atmosphere, the people around. I think it's something incredible."

Her next opponent will be Roland Garros champion Jelena Ostapenko in the second round. Russian Daria Kasatkina put out 2012 winner Agnieszka Radwanska 7-5, 6-4, with the 31st-ranked Pole still not back to peak performance after an injury-plagued 2017.

Meanwhile, second seed Muguruza remains keen for another taste of life at the top of the WTA. "It is important," the Spaniard said on Tuesday at the Aviation Club. "I always have it in my mind, to be able to fight for that. It was a great feeling to be there (No. 1), even though it's tough. I know it's the hardest one, but I'm going to be there, try to."

Wimbledon winner Muguruza said that after reaching the WTA summit, her intensity to play for ranking may have tapered off — but she keeps the goal at the top of her Tennis agenda. The WTA top ranking was a revolving door in 2017, with five women holding the spot during the season.

"When you're No. 1, you feel like every tournament you play, you kind of almost have to win it. You have to always maintain a super-high level for everyone you play because everybody's playing super-relaxed, with nothing to lose. You feel a lot of pressure, there are a lot of things to control."

The 24-year-old added: "You realize a lot of things when you're there. You're like, 'I'm here, I can't go more up, all I can do is go down. There's all these girls that want my spot, I have to defend it. You have this, like, alarm all the time.'"

Muguruza, seeded to meet holder Elina Svitolina in the final, said she is less stressed by her ranking now that she's been to the top and back. "I'm trying to be a little bit less (intense). Now (today's) type of game and players, it's going to be a lot of movement. I don't want to be thinking every tournament what are the options for this and for that. Because I was already number one last year, it's less dramatic. I always know there's a chance. That's fine for me."

Cilic, Ramos Vinolas and Verdasco win at Rio

World No. 3 Marin Cilic of Croatia won his debut at the Rio Open. The tournament's top seed beat Argentinian Carlos Berlocq 6-3, 6-2 on Monday.

Fourth seed Albert Ramos Vinolas of Spain overcame Brazilian Rogerio Dutra Silva in three sets: 6-3, 3-6, 6-4 and Spaniard Fernando Verdasco also needed three sets to win. He qualified to the ATP 500 round of 16 with a 6-2, 3-6, 6-3 victory over Argentina's Leonardo Mayer.

Second seed and world No. 6 Dominic Thiem of Austria will debut on Tuesday in the clay tournament at Rio de Janeiro's Jockey Club. On Sunday Thiem won the title of the ATP 250 Argentina Open in Buenos Aires for the second time in the tournament that kicks off Latin America's Tennis season. — Agencies