Rafael Nadal is "almost impossible" to beat at the French Open and it is a "big ask" for Dominic Thiem to stop him winning a 12th title on Sunday, says ex-British number one Greg Rusedski.

Spanish second seed Nadal, 33, faces Austrian fourth seed Thiem at 14:00 BST in a repeat of last year's final.

"It is time for Thiem to step up in the biggest test in men's tennis on this surface," Rusedski told BBC Sport.

Thiem is seeking his first Grand Slam title, while Nadal is chasing his 18th.

The Spaniard can become the first player to win 12 singles titles at one Grand Slam.

"Rafa is almost impossible to beat in the five-set format on clay," former US Open finalist Rusedski added.

"The start is huge for Thiem. If he can get one of the first two sets - especially the first - then he has got a shot.

"If Rafa goes up one set early then all the pressure goes on Thiem's shoulders and Rafa relaxes.

"Thiem has to sneak that first set and get off to a brilliant start, then it would get very exciting."

'Heir apparent' Thiem aiming for revenge

Thiem reached his first Grand Slam final at Roland Garros last year, but was outclassed in a 6-4 6-3 6-2 defeat by Nadal on Philippe Chatrier court.

Now the 25-year-old has an opportunity to make amends for that defeat when they return to Roland Garros' newly rebuilt show court.

The world number four has beaten Nadal in three-set matches on the European clay swing in each of the past three years, including a 6-4 6-4 win in the Barcelona Open semi-finals in April.

Nadal leads their head-to-head record 8-4, but Thiem is one of only a handful of players to have beaten the 17-time major winner multiple times on clay.

"He's the heir apparent that everyone has been talking about. But to beat the maestro is a different thing." Rusedski said.

"The expectation isn't on Thiem but he has to perform a lot better than last year's final.

"He's got to throw that match behind him and remember the match in Barcelona where he destroyed Nadal."

How can Thiem trouble 'frugal' Nadal?

Thiem won his first Masters 1000 title - the tier of ATP tournaments below the Grand Slams - by beating Roger Federer at Indian Wells earlier this year and Rusedski believes the Austrian is a much better player than when he met Nadal in the final 12 months ago.

"He's added the sliced backhand, he's playing a little closer to the baseline, he's willing to transition forward as well as using the drop shot," Rusedski said.

"So with that extra variety, and his strength and his power from behind the baseline, he's got a shot.

"I think he also needs to change the variety on his serve to keep Rafa off balance, as well as transitioning sometimes and bringing him in.

"Physically, he also has to stand toe to toe with Rafa. That is something which is demanding mentally and physically.

"Rafa gives you nothing, he is the most frugal man on a tennis court I've ever seen. He is going to play the old tactic he does against Roger Federer all the time, serve into that backhand side, trying to break it down up high and then quick into the forehand corner."