PARIS: Novak Djokovic faces an anxious wait to find out if a knee injury will derail his Olympic gold medal dream after reaching the semi-finals on Thursday as old rival Andy Murray slipped into retirement.
On a dramatic day at Roland Garros, Zheng Qinwen ended Iga Swiatek's 25-match unbeaten streak in Paris to become the first Chinese player to reach an Olympic singles final.
Defending men's champion Alexander Zverev was knocked out and took a bitter swipe at the crammed Games schedule.
Djokovic said he was "hoping for the best" after aggravating his injured right knee, which had required surgery in June, during his 6-3, 7-6 (7/3) win over Stefanos Tsitsipas in the quarter-finals.
The 37-year-old needed on-court treatment and a painkiller after slipping on the baseline of Court Philippe Chatrier.
He recovered from 0-4 and then 2-5 down in the second set, saving three set points in the ninth game, before racing away with the tie-break.
The Serb faces Italy's Lorenzo Musetti on Friday in the last four.
"I'm concerned about the state of the knee. I have to go and examine it with medical staff and then let's see," said Djokovic.
"I'm playing at 7:00 pm tomorrow, which gives me slightly more time. But I'm hoping I can be ready and be optimistic. I have to be."
The 24-time Grand Slam title winner is still looking for a first Olympic gold medal, having so far managed just a bronze in Beijing in 2008.
He is through to his fourth Olympic semi-final.
Musetti stunned Zverev 7-5, 7-5 as the world number 16 became the first Italian to reach the semi-finals since tennis returned to the Olympics at Seoul 1988.
Zverev said illness was partly responsible for his defeat but also blasted the schedule as "a disgrace" after playing his quarter-final less than a day after his last-16 tie.
Alcaraz, back at Roland Garros where he won a maiden French Open in June, became the youngest Olympic semi-finalist since Djokovic in 2008.
The second seed saw off 13th-ranked Tommy Paul of the United States 6-3, 7-6 (9/7) after recovering from a break down in the second set and saving a set point in the tie-break.
"It's all about the fight," said Alcaraz, 21, who was playing the day after he and Rafael Nadal suffered a heartbreaking doubles loss in what was probably the veteran's final appearance at Roland Garros.
Alcaraz will next face Felix Auger-Aliassime, who beat sixth-seeded Casper Ruud, a two-time French Open runner-up, 6-4, 6-7 (8/10), 6-3.
Murray's career came to an end when he and Dan Evans lost to American pair Taylor Fritz and Tommy Paul 6-2, 6-4 in the men's doubles quarter-finals.
Former world number one and three-time Grand Slam title winner Murray, 37, had already announced that the Olympics would be his last event.
"I'm proud of my career, my achievements and what I put into the sport," said Murray.
"Obviously it was emotional because it's the last time I will play a competitive match. But I am genuinely happy just now. I'm happy with how it finished."
Seventh-ranked Zheng triumphed 6-2, 7-5 over world number one Swiatek and will face Donna Vekic in Saturday's gold medal match.
Croatia's Vekic breezed past Anna Karolina Schmiedlova of Slovakia 6-4, 6-0.
"If you ask me to play another three hours for my country, I would," said Zheng, who had played back-to-back three-hour matches to make the semi-final.
Swiatek, 23, a four-time French Open champion at Roland Garros, had defeated Zheng in all of their six previous meetings but she was hit off court by the powerful 21-year-old Australian Open finalist.
Zheng is the first Chinese man or woman to reach an Olympic singles gold medal match, bettering the run of Li Na, who finished fourth in the women's event at Beijing in 2008.
China's only Olympics tennis gold came thanks to Li Ting and Sun Tiantian in the women's doubles at Athens in 2004. --AFP