Shubman Gill lost a few kilos during a battle with dengue fever but was back to his best against Bangladesh as he posted his first World Cup fifty to help hosts India keep their perfect record intact at the tournament.

The opener was rested from India's first two matches against Australia and Afghanistan but slotted back into the side in the third against Pakistan, scoring all 16 of his runs via boundaries.

Against Bangladesh on Thursday, Gill smashed five fours and a pair of sixes in his 53 from 55 balls, while also doing plenty of running between wickets in a rapid-fire opening partnership of 88 runs with captain Rohit Sharma.

"(It) obviously felt nice," the 24-year-old said of his first fifty at a World Cup.

"When I was a little sick, I was feeling a bit disappointed on missing out on those opportunities, but definitely felt nice to get some game-time.

"I think you have to be mentally willing to go out there and that's what I was thinking when I was there," he added of his illness.

"Lost a few kilos in the process but hopefully I'll be able to gain that back."

India, 4-0 at the tournament after thrashing Bangladesh by seven wickets, will be glad to have Gill fit and in form ahead of Sunday's blockbuster against 2019 runners-up New Zealand, the only other team yet to taste defeat.

No player has come close to Gill's 1 407 ODI runs over the past 12 months, and he is ranked second among batters in the format behind Pakistan's prolific skipper Babar Azam.

India have won all four of their matches chasing totals, with captain Rohit setting a cracking pace and Virat Kohli, who scored his 48th ODI ton against Bangladesh, in top form at No 3.

Sandwiched between the skipper and one of India's all-time greats in the batting order, Gill cannot help but soak up a few lessons.

"You can see how confident they are, the way they are scoring runs," he said.

"Definitely, I think this is one of the things which was talked about, especially leading up to the World Cup, that we were not chasing that well, those big totals.

"But I think having won all the four matches while chasing, I think it's a great way to have that momentum with us in those big games that we'll have moving forward."