Miami - America's Mardy Fish hung onto the top national ranking place through his 6-4, 6-3 defeat of South African Kevin Anderson in the Miami Masters on Monday.
The victory, which came only hours after challenger John Isner crashed out in the event, was played on a secondary showcourt.
Fish, eighth in the world, has been complaining that even with his national status, he has yet to play on a showcase court so far this month at Indian Wells and Miami.
Spanish 12th seed Nicolas Almagro beat compatriot Fernando Verdasco 6-3, 6-4 while Frenchman Richard Gasquet defeated Albert Ramos 6-2, 5-7, 6-3.
In the women's draw, Maria Sharapova survived a brief lapse but regained control in time to post a 6-4, 7-6 (7/3) defeat of fellow Russian Ekaterina Makarova for a place in the quarter-finals.
The second seed was cruising when her game went off the boil in a fourth-round match not helped by 11 double-faults in the sun-baked conditions at Crandon Park.
"I was up a set and a break and played a bit sloppy," said the three-time Miami losing finalist who was beaten for the title last year by Victoria Azarenka. "It was a combination of her stepping up and being much more aggressive.
"She had opportunities, she served for the second set. But I just hung in there and stayed consistent. I played a much smarter tie-breaker, and I think that helped me extremely.
"I managed to win the match in two sets, so that's a plus. Consistency will be really important in the next match and getting the percentage up and also returning a little bit better, little things here and there."
Sharapova will next face the winner from China's eighth seed Li Na and German 12th seed Sabine Lisicki.
French seventh seed Marion Bartoli reached the last eight after she beat Russia's Maria Kirilenko 6-1, 6-2.
Latest results on Monday at the $9.6-million Miami Masters:
Women, fourth round:
Maria Sharapova, 2, Russia, def Ekaterina Makarova, Russia, 6-4, 7-6 (7-3)
Agnieszska Radwanska, 5, Poland, def Garbine Muguruza Blanco, Spain, 6-3, 6-2
Serena Williams, 10, US, def Samantha Stosur, 6, Australia, 7-5, 6-3
Marion Bartoli, 7, France, def Maria Kirilenko, Russia, 6-1, 6-2
Men, third round:
Mardy Fish, 8, US, def Kevin Anderson, South Africa, 6-4, 6-3
Nicolas Almagro, 12, Spain, def Fernando Verdasco, Spain, 6-3, 6-4
Richard Gasquet, France, def Albert Ramos, Spain, 6-2, 5-7, 6-3