It’s the first time since 2000 that the American has not reached the quarterfinals of the Miami event. The world No. 1 was trying to win her fourth straight and ninth title in Key Biscayne.
The 34-year-old, 21-time major champion has not won a tournament since winning the event in Cincinnati last August.
“It’s obviously disappointing, but I’ve won here a lot, so it’s okay,” Williams said in her news conference.
“I did the best that I could. My fans, you know, I did the best I could do. I can’t win every match. The players come out and play me like they’ve never played before in their lives.
“You know, the best that I could do today. I have to be 300% every day, so…”
Russian Kuznetsova made only 15 unforced errors to Williams’ 50. Kuznetsova broke serve 6 times. The 15th seed won Miami back in 2006.
“I’m too old,” Kuznetsova said in her on-court interview. “I’ve been many times on this court. I love being back here, and I’m really happy with my performance.”
“Physically I’m fine,” Williams said. “I don’t know, I guess I didn’t move today. Maybe that was one of the things that didn’t work out for me.”
Asked about if there are too many expectations placed on her, Williams said: “I don’t think there is expectations. I mean, there obviously is expectations, but I think overall I put a lot of expectations on myself more than anything.
“So that’s pretty hard to live up to, is the expectations I put on myself.
“I have so many people saying congratulations I feel like I won the title already,” Kuznetsova told media. “Not real.
“Now these days the tennis is extremely tough and each opponent is really difficult to play. I’m just focusing for tomorrow’s match. I don’t have too much time to rest.
“Really looking forward to give my best another day tomorrow.”
“I knew I had to go for my serves. I think I barely did double fault. I knew I had to go aggressive. If I would give some easy second serves I would get my ass kicked, you know, out there.”
Asked if she thought Williams was struggling, she said: “I mean, you say drama when somebody, No. 1, which is probably one of the greatest in the history of athletes didn’t win four slams. This is drama. For me this would be miracle of the year. For us it’s drama that she didn’t win the fourth Grand Slam of the year.
“She struggled a little bit probably because she lost Australian Open, but, I mean, she is still No. 1 and she still plays great. I don’t see much to be depressed about.”
“I mean, going into today’s match I knew she is not on her top game, but neither was I. I didn’t feel good. I knew I was hang in there and I will fight for each ball.
“It’s her game, as I said, mentioned earlier. She goes for her shots, and I have to move her as more I can and make her do more unforced errors. This is my goal, how I have to win, because I’m not going to overhit her or do more winners than Serena, or do more aces. This is not my game.
“If I put myself and Serena on the paper, that’s what I fight for: for her to make unforced errors, to bring more returns back, to serve better, and I think it worked really well for me today.”
In addition to the top seed falling, the No. 2 on the women’s side also lost. 2012 champion Agnieszka Radwanska, lost to No. 19 Timea Bacsinszky 2-6, 6-4, 6-2.
A major upset took place on the men’s side as well when No. 2 Andy Murray lost to No. 26 Grigor Dimitrov 6-7 (1), 6-4, 6-3. Dmitrov won the last five games to close the match.
Murray made 55 unforced errors and had his serve broken six times in the match.
“I got myself up 3-1 in the third set, which is a good position to be in, and then the mistakes started creeping in,” Murray said. “That’s nothing to do with adjustments. A lot were unforced errors and just poor mistakes.”
“I made many more mistakes than usual, especially in the third set, so that was surprising for me.
“But, again, I got myself in a winning position, so it’s not like I came out and played like awful stuff the whole match. Just when you get in winning positions you need to put your foot down. The last two tournaments I certainly haven’t done that.”
“In then the third set managed to get up a break and then a lot of unforced errors. Credit to him. He was more solid than me.”
“I’m happy,” Dimitrov said. “Of course I’m happy with that victory. Every time you beat a top player you know you must have done something good.
“As I said, I’m just very happy with the win and just focusing on the next round. I’m not trying to rank anything right now. I’m still in the tournament. Looking for my next opponent. It’s a good win. Still in a way it’s the beginning of the year. Clay is coming after.
“So just focusing on what I have to do.”
There is only one of the top five men’s seeds left – No. 1 Novak Djokovic.
MIAMI OPEN PRESENTED BY ITAU – MIAMI, UNITED STATES
21 MARCH – 3 APRIL, 2016
RESULTS – MARCH 28, 2016
Women’s Singles – Fourth Round
 S. Kuznetsova (RUS) d  S. Williams (USA) 67(3) 61 62
 A. Kerber (GER) d T. Babos (HUN) 62 36 64
 T. Bacsinszky (SUI) d  A. Radwanska (POL) 26 64 62
 V. Azarenka (BLR) d  G. Muguruza (ESP) 76(6) 76(4)
 S. Halep (ROU) d [WC] H. Watson (GBR) 63 64
 E. Makarova (RUS) d  E. Svitolina (UKR) 61 64
 M. Keys (USA) d I. Begu (ROU) 63 61
 J. Konta (GBR) d  M. Niculescu (ROU) 62 62
Women’s Doubles – Second Round
M. Gasparyan (RUS) / M. Niculescu (ROU) d  M. Hingis (SUI) / S. Mirza (IND) 64 62
 B. Mattek-Sands (USA) / L. Safarova (CZE) d D. Kasatkina (RUS) / E. Vesnina (RUS) 62 63
Men’s Singles – Third Round
 G. Dimitrov (BUL) d  A. Murray (GBR) 67(1) 64 63
 K. Nishikori (JPN) d  A. Dolgopolov (UKR) 62 62
 R. Bautista Agut (ESP) d  J. Tsonga (FRA) 26 63 76(3)
 M. Raonic (CAN) d  J. Sock (USA) 76(3) 64
 G. Monfils (FRA) d  P. Cuevas (URU) 63 64
 N. Kyrgios (AUS) d [Q] T. Smyczek (USA) 64 64
D. Dzumhur (BIH) d [Q] M. Kukushkin (KAZ) 46 61 62
An. Kuznetsov (RUS) d A. Mannarino (FRA) 26 75 60
Doubles – Second Round
 B. Bryan (USA) / M. Bryan (USA) d R. Bautista Agut (ESP) / V. Troicki (SRB) walkover
 P. Herbert (FRA) / N. Mahut (FRA) d A. Qureshi (PAK) / G. Simon (FRA) 62 63
[WC] O. Jasika (AUS) / J. Smith (AUS) d D. Marrero (ESP) / B. Paire (FRA) 63 64
R. Klaasen (RSA) / R. Ram (USA) d [PR] M. Cilic (CRO) / M. Draganja (CRO) 76(10) 60
ORDER OF PLAY – TUESDAY, MARCH 29, 2016
STADIUM start 11:00 am
ATP –  R. Gasquet (FRA) vs  T. Berdych (CZE)
Not Before 1:00 pm
WTA –  T. Bacsinszky (SUI) vs  S. Halep (ROU)
ATP –  N. Djokovic (SRB) vs  D. Thiem (AUT)
ATP –  G. Monfils (FRA) vs  G. Dimitrov (BUL)
Not Before 7:30 pm
WTA –  S. Kuznetsova (RUS) vs  E. Makarova (RUS)
Not Before 9:00 pm
ATP –  K. Nishikori (JPN) vs  R. Bautista Agut (ESP)
GRANDSTAND start 10:50 am
ATP – [LL] H. Zeballos (ARG) vs  D. Goffin (BEL)
ATP –  G. Simon (FRA) vs L. Pouille (FRA)
ATP –  N. Kyrgios (AUS) vs An. Kuznetsov (RUS)
Not Before 5:00 pm
ATP – D. Dzumhur (BIH) vs  M. Raonic (CAN)
COURT 1 start 4:00 pm
WTA –  T. Babos (HUN) / Y. Shvedova (KAZ) vs C. Dellacqua (AUS) / S. Stosur (AUS)
WTA – M. Gasparyan (RUS) / M. Niculescu (ROU) vs V. King (USA) / A. Kudryavtseva (RUS)