Castigliione della Pescaia - Alberto Contador fell on the final dash for the line but retained the Giro d'Italia leader's pink jersey after the sixth stage was won by Germany's Andre Greipel on Thursday.
The Spaniard, who did not lose time on his direct rivals, held the overall race lead but was in such pain he could not pull on the pink jersey at the victory ceremony.
"Alberto needs to undergo tests," Tinkoff team manager Stefano Feltrin said, adding that the Spaniard was notably suffering with his left shoulder and knee.
"First he has to be examined by our doctor Piet Daneels," added Feltrin, who declined to comment on the possibility that the race favourite could be forced to withdraw with Friday's stage, the longest of this year's race, covering 264km from Grosseto to Fiuggi, south of Rome.
Contador, the 2008 champion, is attempting to become the eighth rider to achieve the double of the Giro and the Tour de France in the same year.
He holds a two-second lead on Italy's Fabio Aru with Sky's Richie Porte of Australia third at 20sec in the overall standings.
Contador crashed 200 metres from the line, along with other riders who were brought down in a wave provoked by Italy's Daniele Colli's fall on the other side of the road, which resulted in the Nippo rider suffering a broken arm.
Colli came a cropper when he was struck by a camera zoom lens held by a spectator over the barrier, once again raising questions about security after the race was also interrupted by a collective crash on Sunday provoked by a spectator on a bicycle.
The rules state that the time differences are neutralised in case of a fall or accident in the final three kilometres, except for the summit finishes.
Lotto rider Greipel won a sprint for the line ahead of Italians Matteo Pelucchi and Sacha Modolo after the 183km ride through the Tuscan countryside.
The German champion managed to cross the line just after the fall following 4hr 19min 42sec in the saddle, to finish ahead of the pursuing Pelucchi and Modolo.
A breakaway of five riders in two groups -- first Eduard Michael Grosu and Marek Rutkiewicz, and then Marco Bandiera, Alessandro Malaguti and Alan Marangoni -- opened the race from the first hour.
The peloton however did not let the five build up a lead of more then five minutes and caught up 14 minutes from the line.
It was the third Giro stage win for Greipel, nicknamed the 'Gorilla', after 2008 and 2010, and his fourth victory of the season.
Greipel, 32, has also notched up six stage wins on the Tour de France and four on the Vuelta a Espana.
"I now have the points leader's red jersey but it's not my main objective. What I want is to win stages," said Greipel, who is not among the favourites for Friday's seventh stage which finishes with a climb.
Results on Thursday after the sixth stage of the Giro d'Italia, a 183 kilometres ride from Montecatini Terme to Castiglione della Pescaia:
1. Andre Greipel (GER/LOT) 4hr 19min 42sec, 2. Matteo Pelucchi (ITA/IAM) same time, 3. Sacha Modolo (ITA/LAM) s.t., 4. Manuel Belletti (ITA/SOU) s.t., 5. Giacomo Nizzolo (ITA/TRE) s.t., 6. Alessandro Petacchi (ITA/SOU) s.t., 7. Elia Viviani (ITA/SKY) s.t., 8. Luka Mezgec (SLO/GIA) s.t., 9. Nicola Ruffoni (ITA/BAR) s.t., 10. Davide Appollonio (ITA/AND) s.t., 11. Moreno Hofland (NED/LNL) s.t., 12. Kvin Reza (FRA/FDJ) s.t., 13. Alexander Porsev (RUS/KAT) s.t., 14. Juan Jos Lobato (ESP/MOV) s.t., 15. Grega Bole (SLO/CCC) s.t., 16. Jaroslaw Marycz (POL/CCC) s.t., 17. Michael Matthews (AUS/ORI) s.t., 18. Tom Boonen (BEL/ETI) s.t., 19. Greg Henderson (NZL/LOT) s.t., 20. Patrick Gretsch (GER/ALM) s.t.
23. Richie Porte (AUS/SKY) s.t., 26. Fabio Aru (ITA/AST) s.t., 65. Rigoberto Uran (COL/ETI) s.t., 134. Alberto Contador (ESP/TIN) s.t.
1. Alberto Contador (ESP/TIN) 20h25:36, 2. Fabio Aru (ITA/AST) at 0:02sec, 3. Richie Porte (AUS/SKY) 0:20, 4. Roman Kreuziger (CZE/TIN) 0:22, 5. Dario Cataldo (ITA/AST) 0:28, 6. Esteban Chaves (COL/ORI) 0:37, 7. Giovanni Visconti (ITA/MOV) 0:56, 8. Mikel Landa (ESP/AST) 1:01, 9. Davide Formolo (ITA/CAN) 1:15, 10. Andrey Amador (CRC/MOV) 1:18, 11. Damiano Caruso (ITA/BMC) 1:22, 12. Rigoberto Uran (COL/ETI) 1:22, 13. Yury Trofimov (RUS/KAT) 1:24, 14. Leopold Knig (CZE/SKY) 1:24, 15. Amal Moinard (FRA/BMC) 1:46, 16. Darwin Atapuma (COL/BMC) 1:49, 17. Damiano Cunego (ITA/NIP) 2:12, 18. Simon Clarke (AUS/ORI) 2:21, 19. Jrgen Van den Broeck (BEL/LOT) 2:27, 20. Alexandre Geniez (FRA/FDJ) 2:33
30. Ryder Hesjedal (CAN/CAN) 6:11, 36. Carlos Betancur (COL/ALM) 15:06, 129. Andr Greipel (GER/LOT) 56:22, 190. Manabu Ishibashi (JPN/NIP) 1h30:23.