The fourth seeded Prajnesh won 6-0, 6-3 against the sixth seeded Kazakh in the quarterfinals of the $52,080 hard court tournament.
Nagal, ranked 127 in the world, was leading 6-3 5-7 4-1, when Clarke decided to call it quits in the second round match after playing for over two hours.
"Following the completion of pre-tournament testing at the ATP Challenger event in Prague, one player has tested positive for COVID-19," the ATP said in a tweet.
Bengaluru Open 2020: Four top-100 players to participate; defending champion Prajnesh Gunneswaran to lead Indian challenge
Defending Bengaluru Open champion Prajnesh Gunneswaran, currently the highest-ranked Indian at 123, will lead the home challenge in company of 2017 winner Sumit Nagal
Prajnesh Gunneswaran fought hard before going down against Japanese Taro Daniel in the second round of the Bendigo Challenger event on Tuesday.
Wildfires near Canberra force ATP Challenger, ITF Women's circuit tennis tournament to move 600 kilometres away
The Canberra International tennis tournament scheduled to begin Monday will relocate to a city about 600 kilometers (370 miles) away because of continuing wildfires and smoke haze in the region, officials said.
India's Sumit Nagal beats Argentina's Francisco Cerundolo to enter into semi-final of ATP Challenger Campinas
India's Sumit Nagal sailed into the semifinals of the ATP Challenger Campinas with a hard-fought win over Argentina's Francisco Cerundolo.
Rafael Nadal offers Andy Murray his boat as Briton extends stay in Mallorca with second singles win after hip surgery
Instead of playing in New York, where he won the men's singles in 2012, Andy Murray is rebuilding his singles form by playing on the Challenger Tour for the first time in 14 years in an event which bears the name of Rafael Nadal.
Hamburg Open 2019: India's Sumit Nagal qualifies for maiden appearance in main draw of any ATP 500 tournament
21-year-old Sumit Nagal has been enjoying a good run, having reached the semifinals in five challenger tournaments and getting close to the Top 200 rankings
India faces losing Pune Challenger due to increased financial burden following revamp of ATP Challenger Tour
The Pune Challenger could go off the calendar next year due to additional financial burden, expected to be caused by the revamp of the ATP Challenger Tour.
Yuki has long been India's big hope after winning the junior Australian Open in 2009 and prestigious Orange Bowl in the same year.
Yuki Bhambri clinched his first title of the 2017 season on the ATP Challenger Tour following a come-from-behind win over compatriot Ramkumar Ramanathan in the final of the KPIT-MSLTA Challenger in Pune on Saturday.
The revamped PlayerZone is an extranet portal for ATP players, their support teams, coaches, and others in the men's tennis ecosystem.
It is third title of the season for Paes on the ATP Challenger Tour and second with Shamasdin.
The Indian challenge ended at the $50,000 State Bank of India ATP Challenger Tour here Friday, with Somdev Devvarman falling by the wayside in the semi-final to third-seeded Evgeny Donskoy.
A resurgent Yuki Bhambri won the first ever ATP Challenger singles title of his career after scoring a straight-set win over Israeli fourth seed Amir Weintraub in Fergana, Uzbekistan.