On Saturday, she had a tough time early and was stretched to three sets in her 4-6, 7-6 (2), 6-4 win over American Shelby Rogers.
Kerber has struggled for form in recent months and at times was blasted off court by Rogers' heavy hitting from the baseline.
The Spanish 14th seed took just over an hour to record a 6-2, 6-2 win against Romanian Cirstea, who was Bethanie Mattek-Sands' second-round opponent when the American suffered her horrific knee injury.
Three-time champion Novak Djokovic reached the Wimbledon fourth round for the 10th time yesterday with a 6-4, 6-1, 7-6 (7/2) win over Ernests Gulbis of Latvia. I started getting better and better there. "Comparing to the first couple of matches and the last couple of weeks I think this was the most focused I was on the court".
And the Spaniard is excited to face her German counterpart. "We will see. But for me it was really important to win this match".
"I've been in that situation before and it's tough", Querrey, who a year ago knocked out the defending champion, Novak Djokovic, on his way to the quarterfinals, said after his short workout.
"I'm feeling pretty good", said Muguruza, who was beaten in the 2015 final by Williams.
Vandeweghe, considered by some as a dark horse for the title, beat Alison Riske 6-2, 6-4.
The ninth seed had it far from all her own way against Timea Bacsinszky on Centre Court on Saturday, however, as she showed her battling qualities to fight back from a set down and prevail. However, Leander Paes and Yifan Xu, and Zhaoxuan Yang and Divij Sharan lost their first-round matches late on Friday.
Radwanska next faces Russian eighth seed Kuznetsova, the 2004 USA and 2009 French Open champion, who cruised through with a 6-4, 6-0 win over Slovenian qualifier Polona Hercog.
Rybarikova next meets Croatian qualifier Petra Martic, who marched on with a 7-6 (8-6) 6-1 defeat of Kazakh wildcard Zarina Diyas.
The German was the beaten finalist at Wimbledon previous year but has not been in the same form in 2017 and could lose her place at the top of the rankings at the end of this tournament.
But Kerber had awoken and put a huge amount of pressure on the Rogers serve once again, breaking for the third time in the match to serve at 4-3.