American John Isner upset second-seeded Marin Cilic of Croatia 7-6 (0), 6-3 on Tuesday to reach the Miami Open quarterfinals.

The departure of Cilic, the 2014 U.S. Open champion, leaves fourth-seeded Alexander Zverev of Germany as the highest seed remaining in the draw.

Marin Cilic, of Croatia, eyes a return to John Isner during the Miami Open tennis tournament, Tuesday, March 27, 2018, in Key Biscayne, Fla. Isner won 7-6 (0), 6-3. (AP Photo/Lynne Sladky)

Top-seeded Roger Federer lost his opening match at the tournament, and third-seeded Grigor Dimitrov of Bulgaria lost in the third round.

The 14th-seeded Isner, who reached the Miami Open semifinals in 2015, never faced a break point and made 73 percent of his first serves.

In the quarterfinals, Isner will play 19th-seeded Hyeon Chung, a South Korean who defeated Joao Sousa of Portugal 6-4, 6-3.

 

Sloane Stephens, left, shakes hands with Garbine Muguruza, of Spain, after winning their match at the Miami Open tennis tournament, Monday, March 26, 2018, in Key Biscayne, Fla. Stephens won 6-3, 6-4. (AP Photo/Lynne Sladky)

Stephens, Venus advance to Miami Open quarterfinals

Sloane Stephens' loudest fan kept shouting advice and encouragement from the upper deck Monday, perhaps providing the nudge she needed to shake a post-U.S. Open slump.

Stephens earned her biggest victory since winning last year's Open, beating No. 3-ranked Garbine Muguruza 6-3, 6-4 Monday to reach the quarterfinals at the Miami Open.

"Keep it in the court, girl — that's all you've got to do!" the spectator hollered, and Stephens did. During a post-match on-court interview, Stephens thanked the fan, who has attended her past two matches.

"I have no idea who that is," she said later. "Day before yesterday was the first time I'd ever seen him. But it's entertaining. Obviously he knows tennis — he was shouting all the right things."

Sloane Stephens, returns to Garbine Muguruza, of Spain, during the Miami Open tennis tournament, Monday, March 26, 2018, in Key Biscayne, Fla. Stephens won 6-3, 6-4. (AP Photo/Lynne Sladky)

The fan might have picked a good time to jump on the Stephens bandwagon. She'll next face Angelique Kerber with a shot at reaching the Miami Open semifinals for the first time.

Also advancing was three-time champion Venus Williams, who came from behind for the second day in a row and beat defending champion Johanna Konta 5-7, 6-1, 6-2. The victory was Williams' 64th on Key Biscayne.

"I'm not bad at playing tennis," the 37-year-old Williams said, "so why not keep going?"

Victoria Azarenka, another three-time champion, swept 2012 champ Agnieszka Radwanska 6-2, 6-2.

Stephens claimed her first major title in New York in September and is ranked 12th, but she arrived at Key Biscayne with a record of only 3-4 this year.

Venus Williams reacts after defeating Johanna Konta of Germany 5-7, 6-1, 6-2 during the Miami Open tennis tournament, Monday, March 26, 2018, in Key Biscayne, Fla. (AP Photo/Joe Skipper)

"After winning a Grand Slam, it's not easy to come back right out and keep playing," Stephens said. "I was so excited after winning the U.S. Open, I was living my life to the fullest. Now I'm back to my day job. It's a little bit difficult. It takes a little bit of confidence and a lot of hard work."

On the men's side, qualifier Thanasi Kokkinakis lost in the follow-up to his upset of Roger Federer in the second round. Fernando Verdasco rallied from two mini-breaks down in the decisive tiebreaker and beat Kokkinakis 3-6, 6-4, 7-6 (4).

Kerber advanced by rallying past qualifier Yafan Wang 6-7 (1), 7-6 (5), 6-3.

Stephens won with typically dogged defense, and by attacking Muguruza's weak second serve. The Spaniard has been battling a slump herself, falling to 8-6 in 2018.

Johanna Konta, of Germany, hits a return to Venus Williams during the Miami Open tennis tournament, Monday, March 26, 2018, in Key Biscayne, Fla. (AP Photo/Joe Skipper)

"I missed too many shots, a couple of mistakes I shouldn't have done in important moments," said Muguruza, the 2017 Wimbledon champion. "My serve in general could be better. It was not a big difference, but in the end a few details went to her side."

Stephens started the year by losing her opening match in consecutive tournaments, including the Australian Open. She then spent more than a month away from the tour, but said she was no longer celebrating her U.S. Open title.

"After Australia I really took time to get myself together," she said. "If you put in the work, you're able to get results."

More than one fan is applauding her success at the Miami Open, the hometown tournament for Stephens, a South Florida native.

Danielle Collins hits a return to Monica Puig during the Miami Open tennis tournament, Monday, March 26, 2018, in Key Biscayne, Fla. (AP Photo/Joe Skipper)

Next year the event will move 18 miles north to the Miami Dolphins stadium. Stephens, who first came to Key Biscayne for juniors matches more than a decade ago, said she is sad about the switch — and not just because of tennis memories.

"I'll miss Sir Pizza," she said. "I've been going there since I was 12. The same people still work there, and it's such a gem. I'm definitely going to miss that place."

Other winners included American Danielle Collins defeating Monica Puig of Puerto Rico 3-6, 6-4, 6-2 and Jelena Ostapenko topping Petra Kvitova 7-6 (4), 6-3.

Denis Shapovalov edged Sam Querrey 6-4, 3-6, 7-5 and Nick Kyrgios defeated Fabio Fognini 6-3, 6-3.

Alexander Zverev stopped David Ferrer 2-6, 6-2, 6-4 and Frances Tiafoe led Tomas Berdych 6-7 (2), 6-2, 5-4 before the match was suspended at 2:13 a.m. because of rain.

The winner will play Kevin Anderson later on Tuesday.