By Julien Pretot and Shrivathsa Sridhar

PARIS (Reuters) -The fickle Paris crowd made themselves heard on the opening day of the French Open as they booed when Ukraine's Marta Kostyuk refused to shake hands after her first-round defeat by Aryna Sabalenka of Belarus on Sunday.

Stefanos Tsitsipas, the runner-up in Paris in 2021, was dragged into a dogfight by Czech Jiri Vesely before advancing in four sets and seeds Karen Khachanov and Hubert Hurkacz also reached the second round.

Kostyuk had warned she would not shake hands with Russian and Belarusian players due to Moscow's invasion of its neighbour and she kept her word on a sun-kissed court Philippe Chatrier after losing 6-3 6-2 to world number two Sabalenka.

Sabalenka, who initially thought that the boos were for her, theatrically bowed to supporters before thanking them for their backing and later said she understood Kostyuk's choice to skip the customary gesture.

She added the Ukrainian did not deserve to leave the main showcourt in the manner she did, but Kostyuk urged Sabalenka to take a stronger, more personal stand against the war.

"She (Sabalenka) never says that she personally doesn't support this war, and I feel like journalists should change the questions you ask these athletes because the war is already there," Kostyuk said.

"It's been 15 months since the war has begun."

Fifth seed Tsitsipas was made to work hard by Vesely, who is on the comeback trail following a lengthy injury absence, before winning 7-5 6-3 4-6 7-6(7).

"I said to myself there's no chance this is going to a fifth set," said Tsitsipas. "Jiri was a difficult obstacle. He gave me a hard time and I'm happy I overcame it in such a fashion."

Khachanov beat local favourite Constant Lestienne 3-6 1-6 6-2 6-1 6-3 and the 11th seed was joined in the second round by 13th seed Hurkacz who battled past David Goffin 6-3 5-7 6-4 2-6 6-4.

EVANS OUT

Australian wildcard Thanasi Kokkinakis pulled off an upset by downing 20th seed Dan Evans 6-4 6-4 6-4.

Earlier, spectators lined up in huge numbers under panama hats and umbrellas as the year's second Grand Slam tournament started in bright sunshine at Roland Garros.

After a record 50,000 people - compared to 29,000 last year - attended the qualifying event this week, Sabalenka and Kostyuk opened proceedings on centre court as the fashionably late fans queued to get their seats.

With temperatures at 22 degrees and expected to reach 26 in the afternoon, the water vending stands were busy.

There will be an unfamiliar feel to this year's tournament, however, with defending champion and 14-times winner Rafa Nadal out injured.

"Of course, it's a big miss. My son really wanted to see Nadal but now we will be supporting (Carlos) Alcaraz," said Gerald Furst, who runs a tennis club in the south of France.

"I like (Novak) Djokovic as well, he's an amazing player. I believe it'll be a semi-final between Alcaraz and Djokovic. We want to see this match."

Serbian Djokovic is bidding for his third French Open title.

(Reporting by Julien Pretot and Shrivathsa Sridhar; Editing by Ed Osmond)