JOIN USKei Nishikori will face Stan Wawrinka in the third round of the Madrid Open after battling past Bolivian qualifier Hugo Dellien on Wednesday.
Nishikori needed just over two hours to win 7-5, 7-5 after Dellien saved seven set points during an impressive fightback from 5-1 down in the opening set.
The Japanese will go up against Wawrinka on Thursday for a place in the quarterfinals. The winner could play Rafael Nadal.
Wawrinka beat Argentina's Guido Pella 6-3, 6-4, with a break in each set proving enough for the three-time grand slam champion.
Nishikori's compatriot and world number one Naomi Osaka is through to the quarterfinals of the women's tournament for the first time after she eased past Belarusian Aliaksandra Sasnovich 6-2, 6-3.
Osaka pulled out of the semifinals in Stuttgart last month with an abdominal injury but has said she is no longer feeling discomfort during matches.
The two-time major champion will face either Switzerland's Belinda Bencic and Ukraine's Kateryna Kozlova in the last eight.
A potential semifinal foe for Osaka is world number three Simona Halep, who annihilated Slovakian Viktoria Kuzmova 6-0, 6-0 in 44 minutes.
Federer wins on clay return
Roger Federer shook off a brief bout of pre-match jitters as he returned to clay after a three-year absence, posting a 6-2, 6-3 win over Richard Gasquet on Tuesday.
The 20-time Grand Slam champion needed just 52 minutes to brush Gasquet aside in the second round, but confessed he had to control his emotions during afternoon match strategy sessions with his team.
"I was very calm all day, but two hours before, I was feeling it a bit. But that was the only time," Federer said. "The nerves went away quickly, I'm just happy I started well. I'm glad to be back," said the three-time Madrid champion who last played on clay almost three years ago.
"I've been missing the clay, this is a great return, it's a special night for me," he said.
Federer swept up the opening set in 23 minutes and broke Frenchman Gasquet -- who had defeated him twice on clay -- for 5-3 in the second before serving it out a game later.
Federer has won 18 of 21 meetings with Gasquet, just back from six months out with injury.
"I've trained well, but that's not like a match. The points here are quicker and first matches anywhere can be tricky," said Federer.
"But I felt really natural on the clay tonight."