Spanish reigning grand slam champions Rafael Nadal and Garbine Muguruza were shaken and saddened by Thursday's attack in Barcelona that left 13 people dead and another 100 injured.
"Shattered by what has happened in Barcelona! All my support for the families affected and the city," Nadal tweeted.
Nadal's scheduled third-round match against countryman Albert Ramos-Vinolas at the WTA and ATP Cincinnati Masters was postponed due to rain.
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The 31-year-old winner of 10 French Open crowns captured the ATP title this year at Barcelona as well as in Madrid, Monte Carlo and Roland Garros for his 15th grand slam crown in June.
A van driver slammed into crowds on Las Ramblas, Barcelona's most popular street, in a daylight attack claimed by the Islamic State jihadist group. Police were treating the incident as a terrorist attack.
Sixth-ranked Muguruza, who captured the Wimbledon title last month, learned of the incident during a two-hour rain delay in her 6-4, 3-6, 7-6 victory over American Madison Keys.
"I couldn't believe it. I have been so many times to that place, so many times. It's like a main street with a lot of shops and people," Muguruza said.
"I was very surprised because you always hear what happened in Paris and in London. But now, I've been so many times to this place, it's like my home.
"I've been there so many times. So it really shocked me. Really shocked me.
"I'm just sad for the people that were there. I feel like if you're in the wrong place at the wrong time, it can happen anywhere."
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Once Muguruza learned of the attack, she called to make certain her family was unharmed.
"During the rain delay. I'm like OK, I was listening to music, I checked for news and stuff, and I'm like, 'Oh,'" Muguruza said. "I checked with my family. Everything is OK, but, you know, it's just incredible."
Muguruza said she was able to set aside the emotional shock and return to finish her match, deadlocked 2-2 in the third set when the storm struck. She saved three match points before breaking in the 12th game, then led all the way in the tie-breaker.
"I'm used to putting so many things aside to be able to compete. That's my every day, putting things aside and going out there and have two hours of concentration of tennis," Muguruza said.
"I just make sure that people around me are fine. There is not a lot I can do anymore. Just support the people. Be with them. But at the end I've got to go play. And after, we can talk again. But I have to kind of block it for two hours."
David Ferrer, who also advanced to the quarter-finals, tweeted after his match: "All my solidarity with the people affected in the attack of Barcelona .. Very sad all that is happening."
And Ramos-Vinolas tweeted ahead of his meeting with Nadal: "Great pain when seeing what is happening in #Barcelona. A part of me is today with you. A hug to all affected families."