Former world number one Serena Williams beat Alicia Molik 6-2 7-6 to put the number two seeds ahead and Mardy Fish edged out Peter Luczak 7-6 7-6 to seal victory. "I served really well today," Williams told reporters. "Alicia has one of the best serves in women's tennis but I think I returned well and everything is coming together well." Williams, who did not concede a single break point in the whole match, ripped through the first set in less than half an hour.
Australian Molik, who reached a career-high ranking of eighth in 2005 before an inner-ear infection put her out for six months, saved break points in the ninth and 11th games to force a tiebreak in the second set.
Molik then saved two match points at 6-5 and 7-6 but a double fault gave Williams victory at the third time of asking.
In the other Group B match, the Czech Republic gave themselves a first win of the week as they took an unassailable 2-0 lead over India.
World number 23 Lucie Safarova held off the challenge of Sania Mirza to put the Czechs ahead with a 6-2 4-6 6-3 victory.
Tomas Berdych, who, like his girlfriend Safarova, struggled with a virus early in the week, then beat Rohan Bopanna 6-4 6-0.
Later on Thursday, top seeds Serbia can seal their place in the final by beating Argentina. France play Taiwan in the other Group A match.