Miami - Japanese sixth seed Kei Nishikori cruised into the fourth round at the ATP and WTA Miami Open on Monday by routing Ukraine's 27th-seeded Alexandr Dolgopolov 6-2, 6-2.
The 26-year-old Asian number one needed only 72 minutes to advance into a matchup for a quarter-final berth against the winner of a later match between French ninth seed Jo-Wilfried Tsonga and Spanish 17th seed Roberto Bautista Agut, already a champion at Auckland and Sofia this year.
"I played a very solid match," Nishikori said. "It's not easy when you're in the top 10 and everybody is playing their best against you. I have to really stay focused and play my tennis all the time. I think I'm doing well."
Dolgopolov broke Nishikori to open the match but the second of his five double faults in the set cost him a break in the next game. He was broken again in the fourth and eighth games, sending a forehand long with his 19th unforced error to hand Nishikori the last break and the set.
Nishikori broke for a 3-1 lead in the second set and again to close out the match when Dolgopolov netted a backhand on the last point, his 30th unforced error of the match.
Nishikori, who won his fourth consecutive Memphis title last month for his 11th career ATP crown, could face British second seed Andy Murray in the quarter-finals.
Murray, trying to capture his third Miami trophy and reach the final for the fourth time in five seasons, plays Bulgarian 26th seed Grigor Dimitrov in a later match.
On the women's side, Romanian fifth seed Simona Halep, the 2014 French Open runner-up, reached the quarter-finals with a 6-3, 6-4 victory over 69th-ranked British wildcard Heather Watson.
Halep, a first-round loser to China's Zhang Shuai at the Australian Open, next faces either Polish world number two Agnieszka Radwanska, the 2012 Miami champion who took her 18th WTA title earlier this year at Shenzhen, or 20th-ranked Swiss Timea Bacsinszky.
World number one Serena Williams, the three-time defending champion seeking her ninth career Miami title, faces Russian 15th seed Svetlana Kuznetsova, a two-time Grand Slam winner, in a later fourth-round match.
Williams, a 21-time Grand Slam champion, has lost back-to-back finals for the first time since 2004, falling to Germany's Angelique Kerber at the Australian Open and Belarus' Victoria Azarenka at Indian Wells.
Second seed Kerber or 13th seed Azarenka could meet Williams again in the Miami final, but Azarenka meets Spanish fourth seed Garbine Muguruza and Kerber faces Hungary's 49th-ranked Timea Babos in later matches to decide quarter-final berths.
Scores on Monday from the seventh day of the ATP and WTA Miami Open (x denotes seeded player):
Kei Nishikori (JPN x6) bt Alexandr Dolgopolov (UKR x27) 6-2, 6-2
Andrey Kuznetsov (RUS) bt Adrian Mannarino (FRA) 2-6, 7-5, 6-0
Roberto Bautista Agut (ESP x17) bt Jo-Wilfried Tsonga (FRA x9) 2-6, 6-3, 7-6 (7/3)
Grigor Dimitrov (BUL x26) bt Andy Murray (GBR x2) 6-7 (1/7), 6-4, 6-3
Nick Kyrgios (AUS x24) bt Tim Smyczek (USA) 6-4, 6-4
Gael Monfils (FRA x16) bt Pablo Cuevas (URU x23) 6-3, 6-4
Damir Dzumhur (BIH) bt Mikhail Kukushkin (KAZ) 4-6, 6-1, 6-2
Milos Raonic (CAN x12) bt Jack Sock (USA x22) 7-6 (7/3), 6-4
Simona Halep (ROM x5) bt Heather Watson (GBR) 6-3, 6-4
Timea Bacsinszky (SUI x19) bt Agnieszka Radwanska (POL x3) 2-6, 6-4, 6-2
Svetlana Kuznetsova (RUS x15) bt Serena Williams (USA x1) 6-7 (3/7), 6-1, 6-2
Johanna Konta (GBR x24) bt Monica Niculescu (ROM x32) 6-2, 6-2
Victoria Azarenka (BLR x13) bt Garbine Muguruza (ESP x4) 7-6 (8/6), 7-6 (7/4)
Ekaterina Makarova (RUS x30) bt Elina Svitolina (UKR x12) 6-1, 6-4
Madison Keys (USA x22) bt Irina-Camelia Begu (ROM) 6-3, 6-1