Williams, seeded 25 and chasing a 24th Grand Slam title, will face Russian qualifier Evgeniya Rodina for a place in the quarter-finals.
In each of her first two matches this week, Williams dropped the opening set before eventually winning in three.
Ninth-seeded American John Isner reached fourth round for the first time by beating world No.98 Radu Albot of Moldova 6-3 6-3 6-4 earlier on Friday.
Serena will play world number 120 Rodina if she advances to the fourth round.
Serena, who is seeded 25th, had 13 aces en route to her 89th career match at Wimbledon, which ties her older sister for third at the All England Club in the Open Era.
The 38-year-old American - last year's runner-up at Wimbledon - was the oldest entrant in the women's singles field. "So it will be great to play against her".
Keys lost nine games on the trot from 5-2 up in the first set to 0-4 down in the second.
John Isner, the USA ninth seed, saved two match points, fired 64 aces and 88 winners to defeat Belgian qualifier Ruben Bemelmans 6-1, 6-4, 6-7 (6/8), 6-7 (3/7), 7-5.
"It's a wonderful feeling walking out there, I take every match one at a time, I try to put on a good match, try hard every time, and will never underestimate anybody - that keeps me in check".
In the first point of the breaker, Buzarnescu ended a rally by plopping a forehand into the net, and after Pliskova followed up by holding her two service points for 3-0, the set was slipping away from the Romanian.
"I felt much more confident in my game, I served better and I felt so pumped", said Pella, who reached the third round of a major for the first time. I thought I did very well today.
He will next face Mannarino after the French player held off a fightback from Daniil Medvedev to progress 6-4 6-3 4-6 5-7 6-3.