Former US Open champion Bianca Andreescu left the court in a wheelchair at the Miami Open on Monday after sustaining a lower left leg injury that forced her to retire from her last 16 meeting with Ekaterina Alexandrova.
After the opening set was hit by a two-hour rain delay, Canadian Andreescu went down while moving across the baseline during the third game of the second set, grabbing her ankle and screaming in pain.
Andreescu was in tears as the broadcast picked up the 2019 Flushing Meadows champion screaming: "I've never felt this kind of pain before", when the medical staff arrived to treat her.
She was eventually helped to her feet and shared a hug with Russian Alexandrova before leaving Hard Rock Stadium in a wheelchair to a standing ovation from the crowd.
Alexandrova advanced 7-6(0) 0-2 (ret.) and will next face Petra Kvitova, who defeated Varvara Gracheva 7-5 7-6(5).
"I'm just really sorry that it happened to (Andreescu)," Alexandrova said. "Seeing her on the court in so much pain, it's just painful to watch.
"You cannot help, you just can do nothing, which is terrible. And I think she's going to be fine soon, and I'm wishing for her speedy recovery."
Australian Open champion Aryna Sabalenka needed just over an hour to beat former French Open winner Barbora Krejcikova 6-3 6-2, while Elena Rybakina beat Elise Mertens 6-4 6-3.
Rybakina, who won the Indian Wells crown in California, would become only the fifth woman to win the 'Sunshine Double' if she triumphs in Miami.
'Like a vacation'
Earlier, Stefanos Tsitsipas beat Chilean qualifier Cristian Garin 6-3 4-6 6-4 to set up a meeting with Russian 14th seed Karen Khachanov, who won 6-2 6-4 against Czech Jiri Lehecka.
Tsitsipas, still dealing with an ongoing shoulder issue and playing his first match of the Miami fortnight following a bye and walkover, respectively, in the opening two rounds, had 12 aces and saved three of four break points in the two-hour match.
"I was waiting for a very long time to get out there and play," said Tsitsipas. "It almost felt like a vacation this last week, staying in Miami, so I am glad I got started.
"I won't lie, it was difficult out there against an opponent that has shown good tennis against top players in the past."
The match remained on serve until Tsitsipas broke for a 5-3 lead in the opening frame before going on to hold at love to grab the first set without facing a break point.
Tsitsipas was tested early in the second set when he fell behind 0-40 and needed to win five consecutive points to hold serve and draw level at 1-1 but could not repeat that escape at 4-5 as Garin broke to love to force a decider.
The Greek squandered two break point chances in the opening game of a third set but got the break he would need at 4-4 when Garin double-faulted while at 40-30 in a game the Chilean went on to lose before Tsitsipas served out the match.
Argentine Francisco Cerundolo stunned fifth seed Felix Auger-Aliassime 6-2 7-5, while Frenchman Adrian Mannarino knocked out eighth seed Hubert Hurkacz 7-6(5) 7-6(0) and American Frances Tiafoe lost 6-3 6-4 to Lorenzo Sonego.
In other women's matches, third seed Jessica Pegula beat Poland's Magda Linette 6-1 7-5 in a match where she broke four times during a 28-minute first set and then erased a 5-2 double-break to avoid being pushed into a decider.
Pegula, a Miami semi-finalist last year who lives about 30 minutes from the tournament venue, has not dropped a set over her first three matches and will next face Russia's Anastasia Potapova, a 6-4 7-6(4) winner over China's Zheng Qinwen.
Italian 25th seed Martina Trevisan enjoyed a 6-3 6-3 win over former French Open champion Jelena Ostapenko, the Latvian 24th seed, to reach the quarter-finals of a WTA 1000 event for the first time.